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Bulletin  288  June,  1927     V7 

AM 

(ttmutrrttrut  Agricultural  iExurrtmeut  sytatum 

Npw  Hftatrnt,  (£imti« rtintt  C-  o  N  [M 

's 

^3 

The  Biology  of 
THE  BIRCH  LEAF  SKELETONIZER 

Bucculatrix  canadensisella,  Chambers 

Roger  B.  Friend 


CONTENTS 


Page  Page 

Introduction    395       Food  Plants  445 

History    395  Factors    Affecting   Abundance  448 

Systematic  Position   309  Geographical   Distribution    . . .   455 

Laboratory  Methods    399  Effect  of  Temperature  on  De- 
Morphology    400          velopment    458 

Life  History  and  Habits   424       Control     482 

Determination  of  the  Number  Summary     482 

of  Instars    442       Bibliography     483 


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CONNECTICUT  AGRICULTURAL   EXPERIMENT  STATION 


OFFICERS  AND  STAFF 

as  of 
June  1927 


BOARD  OF  CONTROL 
His  Excellency,  John  H.  Trumbull,  ex-officio,  President 

Charles  R.  Treat,  Vice-President  Orange 

George  A.  Hopson,  Secretary  Mount  Carmel 

Wm.  L.  Slate,  Director  and  Treasurer  New  Haven 

Joseph  W.  Alsop Avon 

Elijah  Rogers Southington 

Edward  C.  Schneider Middletown 

Francis  F.  Lincoln    ■ Cheshire 

STAFF. 
E.  H.  Jenkins,  Ph.D.,  Director  Emeritus. 

Wm.  L.   Slate,  B.Sc,  Director  and  Treasurer. 

Miss  L.  M-   Brautlecht,  Bookkeeper  and  Librarian. 

Miss  J.  V.  Berger,  Stenographer  and  Bookkeeper. 

Mrs.  R.  A.  Hunter,  Secretary. 

G.  E.  Graham,  In  charge  of  Buildings  and  Grounds. 

E.  M.  Bailey,  Ph.D.,  Chemist  in  Charge. 

C.  E.  Shepard  "J 

Owen  L.  Nolan  L     ,  _, 

Harry  J.  Fisher,  A.B.    f  Assistant  Chetntsts. 

W.  T.  Math  is  J 

Frank  C.  Sheldon,  Laboratory  Assistant. 

V.  L.  Churchill,  Sampling  Agent. 

Miss  Mabel  Bacon,  Stenographer. 

T.  B.  Osborne,  Ph.D.,  Chemist  in  Charge. 
H.  B.  Vickery,  Ph.D.,  Biochemist. 
Miss  Helen  C.  Cannon,  B.S.,  Dietitian. 

G.  P.  Clinton,  Sc.D.,  Botanist  in  Charge. 

E.  M.  Stoddard,  B.S.,  Pomologist.. 

Miss  Florence  A.  McCormick,  Ph.D.,  Pathologist. 

George  L.  Zundel,  M.S. A.,   Graduate  Assistant. 

A.  D.  McDonnell,  General  Assistant. 
Mrs.  W.  W.  Kelsey.  Secretary. 

W.   E.  Britton,   Ph.D.,  Entomologist  in  Charge;    State 
Entomologist. 

B.  H.  Walden,  B.Agr.     ) 

M.  P.  Zappe,  B.S.  {■  Assistant  Entomologists. 

Philip  Garman,   Ph.D.    ) 
Roger  B.  Friend,  Ph.D.,  Graduate  Assistant. 
John  T.  Ashworth,  Deputy  in  Charge  of  Gipsy  Moth  Work. 
R.  C.  Botsford,  Deputy  in  Charge  of  Mosquito  Elimination. 
Miss  Grace  A.  Foote,  B.A.,  Secretary, 

Walter  O.  Filley,  Forester  in  Charge. 

H.  W.  Hicock,  M.F.,  Assistant  Forester. 

J.  E.  Riley,  Jr.,  M.F.,  In  charge  of  Blister  Rust  Control. 

H.  J.  Lutz,  M.F.,  Assistant  Forester  on  Purnell  Project. 

Miss  Pauline  A.  Merchant,  Stenographer. 

Donald  F.  Jones,  S.D.,  Geneticist  in  Charge. 
W.  R.  Singleton,  S.M.,  Assistant  Geneticist. 
H.  R.  Murray,  B.S.,  Graduate  Assistant. 


Administration. 


Chemistry. 
Analytical 
Laboratory. 


Biochemical 
Laboratory. 

Botany. 


Entomology. 


Forestry. 

Plant   Breeding. 
Soil   Research. 


Tobacco   Sub-station 
at  Windsor. 


M.  F.  Morgan,  M.S.,  Investigator. 
H.  G.  M.  Jacobson,  M.S.,  Assistant. 
Evelyn   M.  Gray,  Stenographer. 

Paul  J.  Anderson,  Ph.D.,  Pathologist  in  Charge. 
N.  T.   Nelson,  Ph.D.,  Assistant  Physiologist. 
T.   R.   Swanback,  B.S.,  Scientific  Assistant. 


THE  TUTTLE,    MOREHOUSE   *   TAYLOR   COMPANY 


The  Biology  of 

THE  BIRCH  LEAF  SKELETONIZED 
Bucculatrix  canadensisella,  Chambers 

Roger  B.  Friend 

I.     Introduction 

The  biology  of  Bucculatrix  canadensisella,  or,  as  it  is  more 
commonly  called,  the  birch  leaf  skeletonizer,  is  known  to  only  a 
very  slight  extent.  Not  only  does  the  insect  have  peculiar  habits 
and  a  specific  structure,  but  its  great  abundance  during  certain 
years,  coupled  with  its  habit  of  feeding  on  native  birches,  renders 
it  of  interest  economically  as  well  as  biologically.  In  the  follow- 
ing pages  are  the  results  of  investigations,  made  during  the  years 
1924,  1925,  and  1926,  into  its  habits,  reactions,  distribution, 
history,  and  morphology.  The  work  is  not  complete,  but  it  is 
intended  that  the  gaps  shall  be  filled,  in  part  at  least,  in  the  future. 

I  am  indebted  to  Professor  Alexander  Petrunkevitch  of  Yale 
University  and  Dr.  W.  E.  Britton  of  the  Connecticut  Agricultural 
Experiment  Station  for  criticism  of  the  work ;  to  Professor  G.  C. 
Crampton  of  the  Massachusetts  Agricultural  College  for  assist- 
ance in  certain  details  of  the  morphological  part ;  to  Messrs.  A. 
B.  Gahan,  R.  A.  Cushman,  and  C.  F.  W.  Muesebeck  of  the  United 
States  Department  of  Agriculture  for  determining  the  species  of 
parasites;  to  Dr.  Annette  F.  Braun  of  the  University  of  Cincin- 
nati for  some  notes  on  the  geographical  distribution ;  to  Mr.  C.  B. 
Hutchings  of  the  Entomological  Branch,  Canada,  for  the  use  of 
an  unpublished  manuscript,  and  to  Mr.  B.  H.  Walden  of  the 
Connecticut  Agricultural  Experiment  Station  for  the  photographic 
work. 

II.     History 

The  earliest  reference  to  the  genus  Bucculatrix  is  found  in  the 
first  volume  of  de  Geer's  "Memoires,"  in  which  is  given  the  life 
history  of  a  "little  caterpillar  with  sixteen  legs,  smooth,  green, 
which  feeds  on  the  lower  side  of  the  leaves  of  Frangula."  It  was 
the  manner  in  which  this  caterpillar  spun  its  cocoon  which  attracted 
the    attention   of    de    Geer,    as    the    following   extract    from    his 


*  This    paper   is    a    dissertation    presented   in    partial    fulfillment    of   the   requirements 
for  the  degree  of  Doctor  of  Philosophy  at  Yale  University. 


396  CONNECTICUT    EXPERIMENT    STATION  BULLETIN    288 

"Memoires"  shows:  "Ouand  elles  sont  parvenues  a  leur  juste  gran- 
deur, ce  qui.  arrive  dans  le  mois  susdit,  elles  filent  contre  les 
feuilles  memes  de  tres-jolies  petites  coques  alongees.  qui  meritent 
extremement  d'etre  connues,  a  cause  de  leur  figure  particuliere. 
Ce  sont  ces  coques  qui  m'ont  determine  a  donner  l'histoire  de  ces 
Chenilles."  He  gives  a  detailed  description  of  the  manner  in 
which  the  cocoon  is  woven,  and  also  gives  brief  attention  to  the 
pupal  and  adult  stages.  There  is  a  plate  of  illustrations  of  the 
larva,  the  structure  of  the  cocoon,  the  adult,  and  the  injury  to  the 
plant.  The  species  described  was  Bucculatrix  frangulella  and  the 
host  plant,  Rhamnns  frangula,  the  buckthorn. 

In  1832  de  Haan  published  a  posthumous  volume  of  Lyonet's 
works  in  which  there  is  a  description  of  a  "chenille  extremement 
petite,  mais  qui  emploie  une  adresse  inconcevable  a  se  filer  une 
coque  cannelee."  This  description  formed  part  of  a  letter  from 
Lyonet  to  Reaumur  written  December  22,  1744,  and  was  later 
sent  to  the  president  of  the  Royal  Society  of  London  to  be  pub- 
lished if  the  society  saw  fit  to  do  so.  Most  of  the  description  is 
devoted  to  the  details  of  the  structure  and  weaving  of  the  cocoon. 
In  his  illustrations  Lyonet  figures  the  larva,  cocoon  and  its  struc- 
ture, and  adult.  The  larvae  were  found  by  Lyonet  on  the  leaves 
of  the  oak.     This  species  was  Bucculatrix  ulmella  Mann  (Zeller). 

The  history  of  the  genus  up  to  1862  is  given  by  Stainton  in  his 
"Natural  History  of  the  Tineina."  Linnaeus  and  Fabricius  neg- 
lected it  entirely,  and  in  1783  Goeze,  in  his  "Entomologische 
Beitrage,"  gave  the  name  Tinea  frangulella  to  de  Geer's  species. 
Neither  de  Geer  nor  Lyonet  gave  names  to  the  species  they 
described.  Retzius,  writing  contemporaneously  with  Goeze,  and 
Villers  six  years  later,  both  gave  different  names  to  the  Tinea 
frangulella  of  Goeze.  The  next  person  after  de  Geer  to  describe 
a  species  of  this  genus  was  Haworth,  who  in  1829  in  "Lepidoptera 
Britannica"  described  Tinea  cuculipenella  with  the  varieties  beta, 
gamma,  and  delta.  Stainton  notes  that  although  Haworth's 
descriptions  are  very  vague,  beta  was  probably  Bucculatrix  boyer- 
ella, gamma,  B.  crataegi,  and  delta,  B.  ulmella  (Lyonet's  species). 
Three  years  later,  in  1832,  appeared  the  posthumous  volume  of 
Lyonet's  works,  in  which  is  described  what  proved  to  be  Buccu- 
latrix ulmella,  as  mentioned  above.  In  1834  Stephens  translated 
Haworth's  description  of  Tinea  cuculipenella  without  mentioning 
the  varieties  gamma  and  delta.  In  1833  Treitschke  had  rede- 
scribed  de  Geer's  species  as  Elachista  rhamnifoliella,  and  a  new 
species,  Elachista  gnaphaliclla.  In  1838  Duponchel  figured  four 
species  in  his  "Lepidopteres  de  France,"  in  the  genus  Elachista, 
namely,  E.  boyerella,  E.  rhamnifoliella,  E.  gnaphaliella,  and  E. 
hippocastanella.  In  1839  Zeller,  in  "Isis,"  placed  the  following 
species  in  section  A  of  his  genus  Eyonetia:  L.  rhamnifoliella  (giv- 
ing reference  to  de  Geer),  L.  albedinella  (boyerella  of  Duponchel), 


BIOLOGY    OF    BIRCH    LEAF    SKELETONIZER  397 

L.  hippocastani  (hippocastanella  of  Dtiponchel),  L.  cristatella,  L. 
higricomella,  L.  cidarclla,  and  L.  crataegi. 

In  1848  Zeller  established  the  genus  Bucculatrix  with  nine  spe- 
cies, the  descriptions  appearing  in  "Linnaea  Entomologica," 
volume  III.  The  nine  species,  with  the  authors  credited  by  Zeller, 
were : 

Bucculatrix   cidarclla  Tischer 
ulmella  Mann 

3.  crataegi  Zeller 

4.  boyerella  Duponchel 

5.  gnaphaliclla  Treitschke 

6.  frangulella  Goeze 
hippocastanella  Duponchel 
nigricomella  Zeller 
cristatella  F.  R. 

The  species  gnaphaliella  had  been  previously  (1839)  placed  by 
Zeller  in  Litliocollctis.  He  included  Bucculatrix  in  a  group  of 
leaf-mining  moths  possessing  eye-caps.  Much  of  the  history  of 
the  genus  from  Zeller  on  does  not  concern  us  here  and  will  be 
omitted.  Stainton,  from  whose  work  much  of  the  above  infor- 
mation has  been  derived,  listed  in  1862  nineteen  species  of  Buc- 
culatrix of  which  he  considered  fourteen  good  and  five  doubtful. 
The  fourteen  were  known  in  the  larval  form  and  their  food  plants 
were  given.  Twelve  of  the  fourteen  are  described  by  Stainton 
very  fully.  This  work  covers  practically  all  that  was  known  of 
the  genus  up  to  the  time  of  writing. 

For  the  earliest  described  American  species  we  must  turn  to  the 
writing's  of  Clemens,  who.  in  the  Proceedings  of  the  Academy  of 
Natural  Sciences,  Philadelphia,  for  i860,  published  the  descrip- 
tions of  four  new  species  of  Bucculatrix:  B.  coronatclla,  B.  pomi- 
foliella,  B.  agnella,  and  B.  trifasciella.  These  descriptions  were 
again  published  in  1872  in  a  posthumous  volume  of  the  writings 
of  Clemens,  edited  by  Stainton.  This  volume  also  includes  a 
description  of  the  genus  by  Clemens.  Chambers,  in  the  Canadian 
Entomologist,  volume  V,  1873,  described  and  mentioned  nine 
American  species  of  this  genus  and  stated  these  to  be  all  the 
described  American  species  known  to  him.  These  nine  are:  B. 
trifasciella  Clemens,  B.  capitcalbclla  n.  sp.,  B.  pomifoliella  Clemens, 
B.  obscurofasciella  n.  sp.  (possibly  synonymous  with  B.  corona- 
tclla Clemens),  B.  luteella  n.  sp.,  B.  agnella  Clemens,  B.  packard- 
ella  n.  sp.,  B.  coronatclla  Clemens,  B.  thuiella  Packard.  Although 
Chambers  considered  his  obscurofasciella  possibly  synonymous 
with  coronatclla  Clemens,  Forbes  (1923)  gives  trifasciella  Clem- 
ens and  obscurofasciella  Chambers  synonymous  with  packardella 
Chambers.  It  is  not  proposed  to  give  a  discussion  of  systematics 
and  synonymy  here,  however.     This  briefly  concludes  the  history 


39&  CONNECTICUT    EXPERIMENT    STATION  BULLETIN    288 

of  the  genus  in  America  up  to  1875,  when  the  species  canadensis- 
ella was  described. 

In  the  Canadian  Entomologist,  volume  VII,  1875,  Chambers 
described  Bucculatrix  canadensis  ell  a,  having  received  his  speci- 
men from  Canada.  This  description  (see  page  401)  concerns 
the  adult  only  and  does  not  mention  the  larva  nor  the  larval  food 
plants.  B.  cidarella  of  Europe  Chambers  considered  close  to  B. 
canadensisella,  although  quite  distinct.  The  larva  of  the  European 
species  demaryella  feeds  on  birch,  but  according  to  the  descrip- 
tion given  by  Stainton  (1862)  it  also  is  quite  distinct  from 
canadensisella. 

For  twelve  years  after  the  description  by  Chambers  there  occurs 
no  mention  of  the  species,  but  in  1887  Lintner  recorded  the  occur- 
rence of  the  insect  in  Monroe  County,  New  York,  where  the 
larvae  were  very  abundant  on  the  leaves  of  Betula  lutea  during  the 
fall  of  1886.  In  1890  Packard  recorded  what  was  in  all  prob- 
ability this  species  on  the  leaves  of  the  white  birch  at  Brunswick, 
Maine.  Lintner  again  reported  it  from  New  York  in  1893,  this 
time  as  injurious  to  all  the  native  birches  in  the  region  of  Ausable 
Forks  during  September,  1891.  The  same  year  Fletcher  stated 
that  all  the  birches  around  Ottawa,  especially  Betula  papyrifera, 
B.  lutea,  and  B.  alba  (European  white  birch)  were  severely 
injured.  From  this  time  on  the  reports  of  the  insect  become  more 
frequent  and  the  injury  caused  by  its  larvae  more  noticed.  Hutch- 
ings  published  a  brief  life  history  in  the  56th  Annual  Report  of  the 
Entomological  Society  of  Ontario  (1926),  and  this  treats  of  the 
insect  more  fully  than  any  other  publication  to  date.  The  species 
is  of  some  economic  importance,  and  most  of  the  literature  on  it 
concerns  the  injury  done  to  the  birch  trees. 

Systematically  the  genus  has  been  neglected,  and  when  men- 
tioned it  is  referred  to  as  aberrant.  Forbes  (1923)  published  a 
key  to  the  species  found  in  northeastern  United  States  with 
descriptions.  For  descriptions  of  species  discovered  in  the  pres- 
ent century  in  America  the  writings  of  E.  Meyrick,  A.  F.  Braun, 
and  A.  Busck  should  be  consulted ;  and  for  Old  World  species 
see  the  writings  of  E.  Meyrick,  especially  his  "Exotic  Micro- 
lepidoptera." 

The  history  of  the  insect  is  interesting  in  view  of  the  fact  that 
at  frequent  intervals  it  appears  in  extraordinary  numbers  and 
severely  attacks  birches  over  wide  areas.  In  1886  Lintner  found 
it  abundant  in  Monroe  County,  New  York,  and  in  1887  it  was 
reported  as  abundant  in  Massachusetts.  During  the  years  1890, 
1891,  and  1892  a  serious  outbreak  occurred  in  Ontario,  New  York, 
and  New  England.  In  1901.  1902,  and  1903  it  was  again  very 
abundant  and  severely  attacked  birches  throughout  this  same  area. 
In  1907  a  small  outbreak  occurred  on  Staten  Island,  New  York, 
and  in  1910  the  insect  was  abundant  at  Kinderhook,  New  York. 


BIOLOGY    OF    BIRCH    LEAF    SKELETONIZER  399 

In  1909  and  1910  birches  in  Minnesota  were  extensively  skeleton- 
ized,  and  the   insect's    depredations   were   severe   in   Ontario   in 

1910,  191 1,  and  1912,  and  in  New  England  in   1909,   1910,  and 

191 1,  growing"  less  serious  in  1912  and  1913.  The  third  outbreak 
of  this  insect  thus  covered  Ontario,  Minnesota,  and  New  England 
between  1909  and  19 12,  with  small  outbreaks  in  New  York  in 
1907  and  1910.  In  1919  the  larvae  were  again  beginning  to 
appear  in  large  numbers.  This  year  they  were  abundant  in  New 
Brunswick  and  were  noticed  in  Connecticut.  In  1920  birches 
were  heavily  skeletonized  and  defoliated  in  Ontario,  Quebec,  and 
New  Brunswick.  In  192 1  the  infestation  continued  in  these 
regions,  and  larvae  were  abundant  in  Minnesota  and  appeared 
commonly  in  Massachusetts.  In  1922  the  injury  to  birch  trees 
was  conspicuous  over  the  Great  Lakes  region  and  in  New  Eng- 
land. This  last  outbreak  began  to  subside  in  1924,  although  the 
larvae  were  injurious  in  Quebec  in  1925.  Beginning  about  1890 
there  have  been  four  serious  outbreaks  of  this  insect,  one  about 
every  ten  years.  Some  of  the  possible  reasons  for  this  periodic 
abundance  will  be  considered  under  the  section  dealing  with  pre- 
daceous  and  parasitic  enemies. 

III.  Systematic  Position 

The  genus  Bucculatrix  was  placed  by  Zeller  in  a  group  of 
minute  leaf -mining  moths  the  adult  antennae  of  which  possessed 
eye-caps.  Along  with  Bucculatrix  were  Lyonetia,  Cemiostoma, 
Nepticula,  etc.  The  first  general  treatise  on  Bucculatrix  placed 
the  genus  in  the  Tineina  (Stainton  1862).  It  is  usually  placed 
in  the  Lyonetiidae  today  and  is  so  classified  by  Forbes  (1923). 
There  are,  however,  differences  of  opinion  as  to  the  classification 
of  Lepidoptera  and  of  this  genus  in  particular.  Thus  Forbes 
places  Bucculatrix  in  the  family  Lyonetiidae  of  the  superfamily 
Tineoidea,  but  Mosher  (1916)  places  it  in  the  family  Bucculatrig- 
idae  of  the  superfamily  Gracilarioidea,  basing  her  decision  on 
pupal  characters;  and  Fracker  (191 5)  places  it  in  the  family  Buc- 
culatrigidae  of  the  Tineoidea.  The  grouping  of  families  and 
genera  in  the  Tineina  is  still  apparently  an  open  question.  The 
genus  will  here  be  placed  in  the  Lyonetiidae  according  to  the  clas- 
sification of  Forbes  and  considered  as  slightly  aberrant.  For  a 
taxonomic  account  of  the  genus  and  a  key  to  the  species  of  north- 
eastern United  States  the  work  of  this  author  may  be  consulted. 

IV.  Laboratory  Methods 

The  life  history  data  were  secured  by  rearing  individual  larvae 
in  glass  jars  or  vials,  each  receptacle  containing  wet  sand  and  a 
fresh    birch    leaf.     Observations    were   made   daily.     Adults    for 


400  CONNECTICUT    EXPERIMENT    STATION  BULLETIN    288 

oviposition  records  were  caged  over  a  birch  twig  in  a  celluloid 
cylinder  with  cloth  ends.  This  permitted  natural  conditions  of 
light  and  air.  The  leaves  were  examined  daily  with  a  glass  and 
eggs  were  marked  with  a  circle  of  black  ink  and  numbered. 
Pupae  were  obtained  by  simply  placing  small  pieces  of  heavy 
cardboard  under  the  plant  in  a  stock  rearing  cage.  The  larvae 
spun  their  cocoons  on  the  under  side  of  the  cardboard.  The 
pupae  were  kept  in  a  box  sunk  in  the  ground  until  the  early  sum- 
mer. Just  prior  to  the  period  of  emergence  they  were  placed 
singly  or  in  groups  of  five  in  glass  vials  plugged  with  cotton  or  in 
large  gelatin  capsules,  the  ends  of  which  were  perforated.  This 
made  observations  on  the  emergence  of  adults  a  simple  matter. 
All  life-history  studies  were  made  in  an  out-door  insectary.  For 
dissecting  fresh  material,  it  was  found  best  to  cover  the  chloro- 
formed specimen  with  a  drop  of  thick  shellac,  add  one  drop  of 
alcohol,  allow  to  set  a  few  minutes,  and  then  immerse  in  saline 
solution.  The  shellac  became  pitchy  and  held  the  insect  firmly, 
but  at  the  same  time  it  could  be  easily  removed  from  the  chitin. 
For  studying  the  external  morphology  the  insects  were  boiled  in 
10  per  cent  potassium  hydroxide  until  clear  and  then  stained  in 
tetrabromfluorescic  acid  twenty-four  hours.  The  chitinized  plates 
stained  deeply  red,  and  the  membranous  cuticle  a  light  pink.  The 
body  being  clear,  the  internal  skeletal  structures  were  readily 
observed.  For  the  temperature  experiments  the  larvae  were  kept 
singly  in  glass  vials  or  in  test  tubes,  the  receptacle  in  either  case 
being  plugged  with  cotton.  The  food  material  was  kept  fresh  and 
unwilted.  The  individual  insects  in  all  cases  were  from  miscella- 
neous field  collections  made  in  the  vicinity  of  New  Haven  except 
where  otherwise  noted. 

V.     Morphology 

The  morphological  descriptions  will  be  confined  to  the  external 
appearance  of  the  various  stages  and  certain  important  anatomical 
details  of  the  exoskeleton.  The  genital  organs  of  the  adult  will 
be  briefly  mentioned  as  they  are  of  considerable  interest  morpho- 
logically and  have  more  or  less  influence  on  the  external  form. 
The  internal  anatomy  is  not  further  described  here,  but  it  is 
intended  that  a  description  of  the  anatomy  and  histology  will  be 
produced  later. 

The  original  description  of  the  genus  by  Zeller  (1848)  is 
reprinted  below. 

Bucculatrix  Zell. 
Elachista  Tr.  Lyonetia  ex  p.  Zell. 

"Caput  lanatum,  comosum. 

"Antennae  breviusculae,  conchula  basali  parvula  instructae. 

"Palpi  nulli ;    os  squamis  epistomii  tectum. 


BIOLOGY    OF    BIRCH    LEAF    SKELETONIZES.  401 

"Alae  anteriores  caudulatae ;  cellula  discoidales  acuta  postice  venulas 
6  emittit ;    vena  subcostalis  longissime  interrupta ;    subdorsalis  simplex  : 

"posteriores  lanceolatae ;  vena  mediana  in  3  ramos  divisa,  subdorsalis 
simplex. 

"Tibiae  posticae  pilosae. 

"Larva  16  pes  supra  epidermidem  foliorum  vivit;  metamorphosis  in  folli- 
culo  affixo  subit." 

The  presence  of  palpi  will  be  brought  out  later,  and  the  vein 
which  Zeller  calls  "mediana"  in  the  hind  wing  is  designated  in 
this  paper  the  radius. 

The  following  is  the  original  description  of  Bucculatrix  cana- 
densis clla  by  Chambers  (1875)  : 


Bucculatrix  canadcnsisclla  n.  sp. 

"The  ornamentation  of  this  species  differs  from  that  of  any  other  yet 
found  in  this  country,  and  though  allied  to  B.  cidarella  of  Europe,  it  is  still 
quite  distinct. 

"Head  white.  Tuft  tipped  with  dark  reddish  brown,  and  the  face  faintly 
tinged  with  purplish  fuscous.  Upper  surface  of  the  thorax  brown  margined 
all  around  with  white.  Base  of  the  fore  wings  white,  followed  by  an 
oblique  brown  fascia,  which  is  nearest  the  base  on  the  costal  margin,  and  is 
followed  by  an  oblique  white  fascia ;  all  of  these  are  placed  before  the 
middle  and  are  followed  by  a  large  brown  patch  which  occupies  the  entire 
wing  to  the  ciliae,  except  that  it  contains  a  white  spot  on  the  middle  of  the 
costal  margin.  The  brown  patch  is  margined  before  on  the  dorsal  margin 
of  the  wing  by  a  small  tuft  of  raised  brown  scales.  At  the  beginning  of  the 
dorsal  ciliae  is  a  white  spot  placed  a  little  before,  but  becomes  almost  con- 
fluent with  a  longer  white  costal  streak.  Behind  these  streaks  to  the  apex 
the  wing  is  pale  brown,  with  a  darker  velvety  brown  apical  spot.  Ciliae 
pale  yellowish,  with  a  dark  brown  hinder  marginal  line  before  their  middle 
not  extending  into  the  costal  ciliae.  Hind  wings  pale  fuscous.  At.  ex. 
$i  inch." 

A.     Adult 
1.     External  Appearance 

As  both  the  above  descriptions  are  rather  brief,  the  external 
appearance  of  the  adult  is  here  given  in  a  little  more  detail.  By 
reference  to  plate  XVII  and  text  figure  12  the  important  mark- 
ings can  be  easily  followed.  Sexual  differences  are  slight  and 
will  be  referred  to  in  the  description. 

The  general  appearance  of  the  adult  in  repose  is  shown  in  plate 
XVII.  The  head  bears  a  dorsal  tuft  of  rather  long  hair-like  scales, 
the  center  of  which  is  brown  and  the  outer  parts  white.  The  "face" 
is  covered  with  gray  or  brownish  scales.  When  the  insect  is  at  rest 
the  head  is  bent  ventrally  so  that  the  labium  touches  the  bases  of 
the  prothoracic  coxae  and  the  short  tongue  is  curled  and  concealed 
between  the  latter.  There  are  no  maxillary  palpi,  and  the  labial 
palpi  are  very  small  and  concealed  beneath  the  head.  The  eyes 
are  black  and  partly  concealed  by  the  scapes  of  the  antennae  which 


402 


CONNECTICUT    EXPERIMENT    STATION 


BULLETIN    21 


are  expanded  to  form  eye-caps.  These  eye-caps  are  white,  and 
from  the  anterior  border  of  each  there  extends  down  in  front  of 
the  eyes  curved  slender  scales  which  give  the  insect  the  appear- 
ance of  having  "shaggy  brows."  The  pedicel  of  the  antenna  is 
short,  and  the  flagellum  contains  29  segments,  each  of  which  bears 
two  whorls  of  brownish  scales.     The  proximal  segments  of  the 


Fig.  12.     Adult  moth,  enlarged  about  ten  diameters. 


flagellum  have  no  scales  on  the  ventral  side.  This  "nude"  area 
is  usually  considered  sensory.  The  first  segment  of  the  flagellum 
(third  of  antenna)  is  longer  than  any  of  the  remaining  segments. 
The  antenna  is  about  two-thirds  the  length  of  the  body  and  fili- 
form. 

The  ground  color  of  the  fore  wings  is  brown,  and  although  typi- 
cally reddish,  it  often  varies  to  a  yellowish.  The  wings  are 
marked  with  transversely  diagonal  white  bars  as  shown  in  figure 
12.  The  basal  bar  is  confluent  with  a  white  area  on  the  meso- 
thorax.  The  second  bar  forms  an  angle  with  the  apex  directed 
distally.  It  sometimes  extends  completely  across  the  wing  and  is 
often  interrupted  in  the  center  by  brown  scales.  The  remaining 
bars  do  not  cross  the  wing  but  terminate  near  the  midline.  There 
are  two  extending  from  the  costal  border  and  one  from  the  anal 
border,  all  three  directed  slightly  apically.  Close  to  the  tip  of  the 
wing  is  another  white  area  whose  exact  size  varies  somewhat  in 
different  individuals.  It  extends  from  the  costal  to  the  distal 
border  of  the  wing  but  does  not  include  the  apex,  this  latter  being 
dark,  almost  sable,  in  color.  There  are  two  other  prominent  dark 
spots  on  the  Aving,  one  at  the  anal  angle  and  one  at  the  distal  mar- 
gin of  the  second  transverse  white  bar.  Both  of  these  are  always 
present,  and  sometimes  there  are  other  dark  spots  on  the  costal 
border.  Beginning  slightly  distal  from  the  middle  of  the  costal 
border  a  row  of  gray  cilia  extends  around  the  wing  almost  to  the 
proximal  end  of  the  anal  border.  The  tuft  of  raised  brown  scales 
on  the  anal  border  of  the  wing  as  described  by  Chambers  is  usually 
conspicuous. 


BIOLOGY    OF    BIRCH    LEAF    SKELETONIZER  403 

The  hind  wings  are  gray  and  their  borders  are  almost  com- 
pletely ciliated.  The  superficial  difference  in  shape  between  the 
fore  and  hind  wings  is  due  to  the  more  extensive  development  of 
scales  on  the  former.  The  scales  on  the  hind  wing  are  less  numer- 
ous and  do  not  project  beyond  the  wing  borders.  Both  the  wings 
are  really  pointed. 

The  dorsal  side  of  the  thorax  is  brown  with  white  areas  later- 
ally, these  latter  being  confluent  with  the  white  basal  areas  on  the 
wings.  Each  tegula  bears  a  group  of  eight  to  ten  bristle-like 
scales  which  extend  along  the  costal  border  of  the  wing  as  far  as 
the  metathorax  when  the  wings  are  folded.  The  pleural  and 
sternal  sides  of  the  thorax  are  silvery-white.  The  coxae  are 
large  and  of  the  same  general  color  as  the  sternum  of  the  thorax 
except  that  the  lateral  borders  are  brownish,  particularly  proxi- 
mally.  The  femora  and  tibiae  are  brown  laterally  and  white 
medially,  as  is  the  first  tarsal  joint.  The  tarsal  joints  two,  three, 
and  four  each  have  a  white  ring  proximally  and  a  brown  ring  dis- 
tally.  The  fifth  tarsal  joint  is  white,  and  its  scales  almost  conceal 
the  tarsal  claws.  At  the  posterior  border  of  the  mesothoracic 
tibiae  at  the  distal  end  is  a  pair  of  spurs,  and  a  pair  of  similar 
spurs  is  found  at  each  end  of  the  metathoracic  tibiae.  There  is 
a  pair  of  spines  at  the  distal  end  of  each  of  the  first  four  tarsal 
joints.  A  row  of  thickly  set  long  hairs  is  found  on  the  anterior 
and  posterior  border  of  the  metathoracic  tibiae. 

The  abdomen  is  covered  with  silvery-white  scales  ventrally  and 
brown  scales  dorsally.  The  males  have  seven  segments  superfi- 
cially distinct  on  the  ventral  side,  the  second  to  the  eighth  inclu- 
sive, and  the  scales  from  the  eighth  practically  cover  the  genitalia. 
The  female  has  six  segments  superficially  distinct  ventrally,  the 
second  to  the  seventh  inclusive.  Scales  from  the  seventh  segment 
conceal  the  border  between  the  seventh  and  eighth,  and  scales 
from  the  latter  cover  the  remainder  of  the  abdomen,  giving  the 
appearance  of  one  broad  segment.  The  ninth  segment  in  the 
female  is  partly  retracted  within  the  eighth,  and  the  tip  of  the 
ninth  projects  very  slightly  beyond  the  scales  of  the  latter.  The 
terminal  fringe  of  scales  on  the  male  abdomen  flares  slightly  but 
never  does  so  on  the  female.  The  female  abdomen  is  slightly 
larger  than  the  male.  On  the  dorsal  side  of  the  abdomen  of  each 
sex  there  are  distinctly  demarcated  eight  segments,  the  first  to 
eighth  inclusive. 

The  body  length  averages  about  three  millimeters  and  the  alar 
expanse  seven  millimeters.     The  sexes  are  of  equal  size. 

2.     Head  (Text  figure  13) 

The  head  is  somewhat  compressed  anterior-posteriorly,  and  the 
occipital  surface  is  flat.     The  antennae  are  filiform  and  composed 


4°4 


CONNECTICUT    EXPERIMENT    STATION  BULLETIN    288 


of  31  joints,  of  which  the  first  or  scape  is  expanded  to  form  the 
eye-cap.  The  second  joint  or  pedicel  is  short  and  subspherical. 
The  third  joint  (first  of  the  flagellum)  is  half  again  as  long-  as 
any  of  those  following'.     The  length  of  the  antennae  compared 


Fig.  13.  Head  and  base  of  antenna  of  adult,  ant  f,  antennal  fossa;  at, 
anterior  arm  of  tentorium;  bt,  base  of  tentorium,  cl,  f ronto-clypeus ;  dt, 
dorsal  arm  of  tentorium;  epicr,  epicranium;  gn,  gena;  gul,  gular  region; 
li,  labium ;  Ip,  labial  palpus  ;  m.v,  maxilla ;  mxp,  maxillary  palpus ;  occip, 
occiput;  ocf,  occipital  foramen;  pg,  postgena ;  pil,  pilif  er ;  prob,  proboscis; 
pt,  posterior  arm  of  tentorium ;    vx,  vertex. 

The  abbreviations  underlined  in  the  figure  are  on  the  posterior  surface 
of  the  head. 


with  the  body  length  is  shown  in  figure  12.  The  eyes  are  black 
and  weakly  spherical.  There  are  no  ocelli.  Between  the  antennal 
fossae  (ant  f )  and  connecting  them  is  the  suture  which  separates 
the  f ronto-clypeus  (cl)  from  the  epicranium  (epicr).  A  suture 
running  between  the  eyes  and  through  the  epicranium  divides  off 
the  vertex  (vx)  anteriorly.     The  vertex  bears  the  forward-project- 


BIOLOGY    OF    BIRCH    LEAF    SKELETONIZER  405 

ing  hairs  of  the  dorsal  tuft  and  is  divided  by  a  median  light  suture. 
The  posterior  part  of  the  epicranium  is  likewise  divided  by  a 
median  suture  and  bears  the  upward-  and  backward-projecting  hairs 
of  the  dorsal  tuft.  The  occiput  (occip)  lies  between  the  epicra- 
nium and  occipital  foramen  and  is  not  sharply  demarcated  from 
the  postgenae  laterally.  The  fronto-clypeus  appears  to  extend 
laterally  to  the  eyes.  The  labrum  is  not  present  as  a  distinct 
sclerite  and  is  represented  by  a  pair  of  pilifers  (pil)  placed  one 
above  each  maxilla.  There  are  no  mandibles.  The  proboscis 
(prob)  is  reduced,  being  about  the  length  of  the  head.  Each 
half  of  the  proboscis  (the  galea)  bears  on  its  anterior  surface  a 
row  of  eleven  papillate  projections  which  appear  pentagonal  in 
cross  section  and  each  of  which  terminates  in  a  short  peg.  The 
particular  function  of  these  was  not  ascertained.  Near  the  base 
of  each  half  of  the  proboscis  and  also  on  the  anterior  surface  are 
three  or  four  setae.  Near  the  base  of  each  maxilla  and  projecting 
from  the  lateral  side  is  a  small  protuberance  (mxp)  which  may 
represent  the  rudiment  of  the  maxillary  palpus.  The  bases  of 
the  maxillae  (rax)  are,  as  usual  with  Lepidoptera,  firmly  fixed 
in  the  ventral  (posterior  in  this  case)  side  of  the  head.  The 
labium  (li)  is  a  small  triangular  sclerite,  with  a  forward-point- 
ing apex,  on  the  ventral  side  of  the  head  and  lies  between  the 
maxillae.  It  bears  a  pair  of  one-jointed  palpi  (lp).  Between 
the  labium  and  the  occipital  foramen  (ocf)  lies  a  gular  region 
(gul)  which  is  bounded  laterally  by  the  maxillae.  Its  separation 
from  the  labium  is  indistinct.  The  postgenae  are  separated  dor- 
sally  from  the  genae  (gn)  by  the  suture  which  divides  the  epi- 
cranium and  ventrally  by  the  sutures  connecting  the  maxillae  with 
the  lower  border  of  the  eyes.  The  genal  regions  are  not  distinctly 
separated  from  the  fronto-clypeus. 

The  tentorium  is  similar  to  that  of  other  Lepidoptera.  The 
body  of  the  tentorium  (bt)  separates  the  occipital  foramen  into 
a  dorsal  and  ventral  part.  The  anterior  arms  (at)  come  for- 
ward from  the  body  and  then  turn  ventrally  to  terminate  at  the 
ventro-lateral  angles  of  the  fronto-clypeus.  The  dorsal  arms  (dt) 
extend  up  from  the  body  to  the  occiput,  bordering  the  foramen 
laterally.  The  posterior  arms  (pt)  extend  down  each  side  of  the 
krwer  part  of  the  foramen  to  the  maxillae.  In  the  figure  of  the 
head  all  the  abbreviations  of  the  parts  on  the  posterior  (mor- 
phologically ventral)  side  of  the  head  are  underlined. 

3.     Cervical  Region  (Text  figure  14) 

The  head  is  supported  by  a  pair  of  laterally  placed  cervical 
sclerites  (cerv)  which  extend  from  the  prothorax.  At  the  cephalic 
end  they  meet  the  body  of  the  tentorium,  and  at  the  posterior 
end  they  articulate  with  the  episterna  and  then  curve  medially  to 
meet  in  the  midline. 


406 


CONNECTICUT    EXPERIMENT    STATION  BULLETIN    288 


4.     Thorax 

The  three  thoracic  segments  are  distinct,  although  the  prothorax 
is  much  reduced.  The  mesothorax  is  the  most  developed,  due  to 
the  development  of  the  fore  wings  and  the  powers  of  flight.  In 
the  following  description  the  nomenclature  of  Crampton  (1909) 
has  been  adhered  to  as  far  as  possible. 


Fig.  14.  Prothorax  of  adult,  anterior  (left)  and  lateral  (right)  aspects. 
bSj  basi-sternum ;  cerv,  cervical  sclerite ;  cxi,  coxa ;  epsi,  episternum ; 
ft,  furca;  lat,  precoxal  bridge;  pat,  patagium ;  prn,  pronotum;  spst,  spini- 
sternum ;    tri,  trochantin. 


a.     Prothorax  (Text  figures  14  and  15) 

The  tergal  region  of  the  prothorax  consists  of  a  central  triangu- 
lar pronotum  (prn)  and  two  laterally  placed  and  conspicuous 
patagia  (pat).  The  apex  of  the  pronotum  meets  the  prescutum 
of  the  mesothorax  in  the  midline. 

The  pleural  region  contains  one  narrow  sclerite,  the  episternum, 
(eps)  which  meets  the  pronotum  above  and  the  coxa  (ex)  below. 
Anteriorly  it  supports  the  cervical  sclerites  and  meets  the  precoxal 
bridge  (lat)  of  the  sternum.  There  is  a  very  minute  sclerite, 
the  trochantin  (tr),  at  the  articulation  of  the  coxa.  The  epi- 
meron  is  obsolete.  From  the  posterior  border  of  the  episternum 
the  pleural  apodeme  extends  into  the  body  cavity  and  meets  the 
arms  of  the  furca  (fx).  Of  the  sternal  sclerites  the  basi-sternum 
(bsx)  is  the  larger  and  extends  laterally  in  the  precoxal  bridge  to 


Fig.  15.  Thorax  of  adult,  dorsal  aspect,  a,  b,  c,  axillary  sclerites; 
anterior  wing  process ;  cost,  costal  sclerite ;  cu,  cubitus ;  ex,  coxa ;  epm, 
epimeron ;  /,  furca ;  fren,  frenulum ;  vise,  median  area  of  scutum ;  pat, 
patagium ;  pph,  postphragma ;  prn,  pronotum ;  psc,  prescutum ;  pstit,  pos- 
terior chitinous  plate  on  metathorax ;  r,  radius ;  teg,  tegula ;  tg  pi,  tegular 
plate  ;  wp,  wing  process  ;  1,  2,  axillary  areas  ;  2nd  a,  3rd  a,  anal  veins.  The 
inferior  numbers  indicate  the  thoracic  segment  to  which  the  part  belongs. 


408  CONNECTICUT    EXPERIMENT    STATION  BULLETIN    288 

meet  the  episterna.  The  median  part  of  the  basi-sternum  is 
folded  and  lightly  chitinized,  extending  slightly  down  the  coxa 
on  each  side.  The  f  urea-sternum  and  basi-sternum  are  not  dis- 
tinctly separated.  Caudally  from  the  furca-sternum  the  narrow 
spini-sternum  (sps^)  extends  along  the  ventral  midline  to  meet  the 
presternal  sclerite  of  the  mesothorax.  What  is  here  called  the 
basi-sternum  is  apparently  the  eusternum  of  other  authors. 
The  furca  extends  up  into  the  body  cavity,  and  from  its  base  a 
pair  of  lateral  arms  extend  to  meet  the  base  of  the  episternum  on 
each  side. 

The  prothoracic  coxa  is  not  divided  into  eucoxa  and  meron, 
although  a'  faint  "suture"  extends  some  distance  up  from  the 
trochanter  along  the  caudo-lateral  surface.  The  coxa  articulates 
freely  with  the  episternum  and  trochantin.  The  trochanter  and 
femur  are  of  the  usual  type.  The  tibia  bears  no  spurs.  The 
coxa,  femur,  and  tibia  are  approximately  equal  in  length,  and  the 
tarsus  is  slightly  longer.  The  basi-tarsus  is  almost  as  long  as 
the  second  and  third  tarsal  joints  combined.  There  are  five  tarsal 
joints  and  each  of  the  first  four  bears  a  pair  of  short  spines  on 
the  posterior  side  of  the  distal  end.  The  tarsus  terminates  in  a 
pair  of  claws. 

b.     Mesothorax  (Text  figures  15  and  16) 

The  tergal  part  of  the  mesothorax  consists  mainly  01  two 
sclerites,  the  large  scutum  (sc2)  and  the  smaller  somewhat  rhom- 
boidal  scutellum  (scl,).  The  anterior  part  of  the  scutum  is 
rolled  in,  and  from  its  margin  is  given  off  the  narrow  prescutum 
(psc2).  This  has  been  considered  a  true  sclerite  by  some  authors, 
but  according  to  Crampton  it  is  simply  a  phragma.  At  its  mid- 
point the  pronotum  is  attached.  At  the  anterior  lateral  angles  of 
the  scutum  are  the  large  tegulae  (teg)  and  beneath  each  tegula 
is  a  smaller  tegular  plate  (tg  pi).  On  each  lateral  side  of  the 
scutum  are  two  pointed  wing  processes  (wp2)  which  project 
slightly  and  which  continue  medially  along  the  under  side  of  the 
scutum  as  chitinous  thickenings.  These  processes  help  support 
the  wing.  The  scutellum  bears  along  its  anterior  and  lateral 
margins  a  phragma  which  projects  slightly  into  the  body  cavity. 
The  posterior  lateral  angles  of  the  scutellum  continue  out  as  the 
anal  borders  of  the  wings.  From  the  posterior  margin  of  the 
scutellum  the  large  postphragma  (pph)  projects  ventro-caudally 
into  the  body  cavity.  This  phragma  is  made  up  of  a  layer  from 
the  mesoscutellum  and  one  from  the  metathorax.  The  layers  are 
easily  separated.  There  is  no  true  postnotum  (or  pseudonotum) 
present  as  a  distinct  sclerite.  The  curved  process  (pwp2)  which 
supports  the  anal  area  of  the  wing  extends  out  from  the  lateral 


BIOLOGY    OF    BIRCH    LEAF    SKELETON1ZER  4°9 

angles  of  the  scutellum.     This  is  called  the  posterior  wing  process 
by  Snodgrass  (1909). 

The  plenron  of   the   mesothorax   is   largely   made   np   of   two 
sclerites,  the  epimeron   (epm2)   and  episternmn    (eps2)   separated 
by  the  vertical  pleural  suture.     The  pleural  apodeme  extends  into 
the  body  cavity  from  this  suture.     The  episternmn  is  divided  into 
a    dorsal    anepisternum    (aneps,)    and    a    ventral    katepisternum 
(kepSo)    separated  by  a  triangular  middle  area.     At  its  anterior 
margin   the   anepisternum    rolls    in    medially.     From    the   dorsal 
margin  of   the  sclerite  the  alar  process    (alp)    projects   upward 
and   supports    the   wing,    and   the    tegular   arm    (tega)    extends 
anteriorly  to  the  anterior  lateral  angle  of  the  scutum  where  it 
abuts  against  the  tegular  plate.     The  tegular  arm  and  alar  process 
together  with  a  ventral  projection   on   the  anepisternum   appear 
to  form  a  single  anchor-shaped  sclerite  fused  with  the  latter  and 
separable  from  it  with  no  great  difficulty.     The  katepisternum 
meets  the  sternum  ventrally.     The  epimeron  is  a  single  undivided 
sclerite  somewhat  membranous  dorsally.     It  meets  the  posterior 
wing  process  and  then  arches  over  as  a  narrow  arm  to  meet  the 
arm  of  the  furca  (f2).     Just  under  the  anal  area  of  the  wing  and 
dorsal  to  the  epimeron  is  the  somewhat  elongate  costal  sclerite 
(cost,).     There  is   no   distinct  trochantin   but   it  may  be   repre- 
sented  by   a   triangular  area   just   over   the   coxa.     The   pleural 
apodeme  widens  at  this  region  and  forms  a  support  for  the  coxa. 
The  anterior  sclerite  of   the  mesosternum    (presternum,   pst2) 
projects  forward  from  the  basi-sternum  (bs2)  to  meet  the  poste- 
rior   sclerite   of    the    prothoracic    sternum    and    extends    slightly 
beyond  it  into  the  body  cavity.     This  sclerite  widens  as  it  meets 
the  basi-sternum.     The  latter  is  triangular,  its  apex  being  poste- 
rior, and  is  divided  by  a  median  longitudinal  suture.     From  this 
suture  and  extending  into  the  body  cavity  is  a  median  chitinous 
blade  (mbl).     Posterior  to  the  basi-sternum  is  the  f urea-sternum 
f  fs2)  which  extends  down  the  medial  side  of  each  coxa  as  a  pedal 
region  (pdr2)  and  holds  the  coxa  rigidly  to  the  body.     The  furca 
(f2)   arises  from  the  f urea-sternum  and  sends   from  its  base  a 
short  curved  process  (fpr2)  into  the  body  cavity  anteriorly.     The 
arms  of  the  furca  meet  the  arms  of  the  epimera  dorsally.     The 
latero-sternites    extend    from    the    basi-sternum    laterally    to    the 
pleural  suture. 

The  mesothoracic  coxa  (cx2)  is  divided  into  an  anterior  eucoxa 
(eucx2)  and  a  posterior  meron  (mer2)  by  a  vertical  suture  on  the 
outer  side.  On  the  .medial  surface  of  the  coxa  lies  a  heavily 
chitinized  angular  plate  (cs2)  which  meets  the  pedal  region  of  the 
furca-sternum.  The  leg  articulates  at  the  trochanter,  the  coxa 
being  immovable.  The  tibia  bears  at  its  distal  end  on  the  posterior 
side  a  pair  of  spurs  of  which  the  outer  is  longer.     The  tarsus  is 


41°  CONNECTICUT    EXPERIMENT    STATION  BULLETIN    288 

similar  to  that  of  the  prothoracic  leg.  The  mesothoracic  leg  is 
slightly  longer  than  that  of  the  prothorax. 

The  wing  venation  (figure  17)  is  much  reduced.  The  subcosta 
and  costa  are  probably  represented  by  the  single  costal  vein.  The 
radius  (r)  is  rather  faint  at  the  base  and  gives  off  five  branches 
distally.  The  median  vein  has  disappeared  except  for  the 
branches  m1  and  m2.  The  cubitus  (cu)  is  single.  There  is  a 
faint  fold  (ista)  which  may  represent  the  first  anal  vein.  The 
second  (2d  a)  and  third  (3d  a)  anals  are  distinct.  There  is  some 
variation  in  the  origin  of  r4,  as  it  sometimes  branches  off  distally 
to  the  position  shown  in  the  figure.  The  costal  vein  bears  a 
retinaculum  (ret)  for  the  frenulum.  The  veins  named  above  are 
according  to  Forbes  (1923). 

The  axial  sclerites  of  the  wing  are  as  shown  on  the  right  side 
of  figure  15.  The  sclerites  a,  b,  c,  3,  and  the  small  sclerite  between 
1  and  3  are  hard  chitinous  plates,  but  those  marked  1  and  2  are 
thickenings  of  the  wing  similar  to  veins.  The  alar  process  of  the 
pleuron  abuts  on  2,  as  does  the  anterior  of  the  scutal  wing-proc- 
esses. The  posterior  of  the  two  scutal  wing-processes  abuts  on 
3,  and  the  posterior  wing-process  supports  a.  The  anal  area  of 
the  wing  folds  along  the  outer  border  of  b. 

c.     Metathorax  (Text  figures  15  and  16) 

The  scutum  (sc3)  of  the  metathorax  is  divided  medially  by  a 
triangular  area  (msc3).  This  does  not  appear  to  be  a  distinct 
sclerite  but  simply  a  more  lightly  chitinized  region.  The  post- 
phragma  of  the  mesoscutellum  is  attached  to  the  anterior  margin 
of  the  scutum,  its  line  of  attachment  extending  to  the  wing  pro- 
cess (awp.,)  at  the  anterior  lateral  angles.  The  scutellum  (scl3) 
is  a  band  stretching  across  the  base  of  the  scutum  and  appears  to 
overlap  the  latter,  due  to  the  presence  of  a  phragma  which  pro- 
jects caudo-ventrally  into  the  body  cavity.  From  the  posterior 
border  of  the  scutellum  a  membrane  drops  ventrally  to  meet  a 
chitinous  arm  which  forms  a  bridge  between  the  ends  of  the 
epimera.  The  center  of  this  bridge  bears  a  chitinous  plate 
(psnt)  to  which  the  tergum  of  the  first  abdominal  segment  is 
attached.  This  represents  the  pseudonotum  (Snodgrass),  al- 
though much  modified  from  a  primitive  condition.  At  its  lateral 
angles  the  scutellum  continues  into  a  narrow  posterior  wing  process 
(pwpa)  which  supports  the  anal  area  of  the  wing. 

The  pleuron  of  the  metathorax  resembles  that  of  the  meso- 
thorax.  The  trochantin  area  at  the  head  of  the  coxa  is  more  dis- 
tinct here,  however.  Dorsally  the  alar  process  (alp)  continues 
directly  with  the  pleural  apodeme,  and  the  anepisternum  (aneps3) 
bears  another  process  which  also  supports  the  wing.  The  costal 
(cost3)    sclerite   is    prolonged   anteriorly   as    a   long   arm.     The 


BIOLOGY    OF    BIRCH    LEAF    SKELETONIZES. 


411 


epimeron  (epm3)  extends  further  posteriorly  than  does  the  same 
sclerite  in  the  mesothorax. 

The  sternum  of  the  metathorax  differs  markedly  from  that  of 
the  preceding"  thoracic  segment.  From  the  central  basi-sternum 
(bs3)  extend  the  narrow  latero-sternites  (not  shown  in  the  dia- 
gram).    The  basi-sternum  extends  caudo-ventrally  to  meet  the 


Fig.  16.  Meso-  (right)  and  meta-  (left)  thorax,  lateral  aspect;  alp,  alar 
process;  ancps,  anepisternum,  bs,  basi-sternum;  cs,  median  coxal  support; 
encx,  eucoxa;  fs,  f urea-sternum ;  fpr,  f ureal  process;  keps,  katepisternum  ; 
mbl,  median  blade ;  mer,  meron ;  pdr,  pedal  region ;  tega,  tegular  arm ; 
sc,  scutum;    scl,  scutellum.     For  other  abbreviations  see  figure  15. 


f urea-sternum  (fs3)  at  the  coxal  support.  There  is  no  pedal 
region  of  the  furca-sternum,  but  the  coxa  is  held  rigidly  by  this 
sclerite  plus  the  basi-sternum.  The  furca-sternum  extends  as  a 
narrow  arm  dorsally  and  then  divides  into  a  furca  (f3).  The 
anterior  f ureal  process  (fpr3)  is  very  large  and  the  furca  is  heavy. 
Dorso-laterally  the  arms  of  the  furca  meet  those  of  the  epimera. 

The  meron  of  the  coxa  is  much  reduced  and  occupies  a  poste- 
rior-medial position,  only  the  eucoxa  being  visible  laterally.  The 
tibia  bears  a  pair  of  spurs  on  the  posterior  side  of  each  extremity, 
and  the  outer  spur  of  each  pair  is  the  longer.  The  leg  is  other- 
wise similar  to  that  of  the  mesothorax. 

The  wing's  (figure  17)  show  greatly  reduced  venation.  There 
are,  besides  the  costal,  three  principal  veins,  the  radius  (r),  cubitus 
(cu),  and  the  second  anal  (2d  a),  the  median  being  represented 


412 


CONNECTICUT    EXPERIMENT    STATION 


BULLETIN    21 


by  two  branches  only.  The  costal  vein  probably  represents  the 
combined  costal  and  subcostal.  The  radius  is  single  and  from  it 
there  branch  the  two'  divisions  of  the  median  (m2  and  m3).  The 
cubitus  is  single.  The  frenulum  (fren)  consists  of  two  stout 
setae  that  are  held  in  the  retinaculum  of  the  fore  wine'. 


Fig.  17.     Fore  (above)  and  hind  (below)  wings;    1st  a,  2nd  a,  3rd  a,  anal 
veins ;  at,  cubitus ;  fren,  frenulum ;  m,  median ;  i-5,  radius ;  ret,  retinaculum. 


The  axial  sclerites  of  the  hind  wing  are  as  shown  in  figure  15. 
An  angular  sclerite  (a)  in  the  anal  region  is  pivoted  on  the  pos- 
terior wing  process.  The  anal  region  folds  along  the  outer  side 
of  this  sclerite.  Two  sclerites  (b  and  c)  lie  between  this  and  the 
anterior  wing  process.  These  three  constitute  the  chitinous  axial 
plates  homologous  with  those  of  the  fore  wing.  The  areas 
marked  1  and  2  are  thickenings  of  the  wing  similar  to  veins  and 
are  homologous  to  the  same  areas  of  the  fore  wing.  The  sclerite 
c  may  correspond  to  3  of  the  fore  wing,  and  the  sclerite  b  to  b 
and  c  of  the  fore  wing.  The  alar  processes  of  the  pleuron  abut 
on  the  area  marked  2,  and  the  subcostal  area  ( 1 )  meets  the  ante- 
rior wing  process  and  the  sclerite  marked  c. 

Snodgrass  (1909)  has  described  the  typical  arrangement  of  the 
axial  sclerites  in  the  wing,  but  the  tracing  of  these  in  the  wing 
here  described  is  uncertain,  due  to  the  difference  in  arrangement, 
and  hence  the  letters  and  numbers  as  given  here  do  not  cor- 
respond to  those  of  the  above  author. 

5.     Abdomen  (Text  figures  18  to  20) 

The  abdomen  has  nine  visible  segments  in  the  female  and  ten 
in  the  male,  although  in  the  latter  sex  the  tenth  is  reduced  to  the 
socii.     The  apical  segments  in  each  sex  are  modified  to  form  the 


BIOLOGY    OF    BIRCH    LEAF    SKELETONIZER 


4'3 


external  genital  apparatus.  The  first  segment  has  a  strongly 
chitinized  tergum,  probably  a  development  in  accordance  with  its 
function  of  supporting'  the  abdomen  on  the  thorax.  The  sternum 
of  the  first  segment  is  indistinguishably  fused  with  that  of  the 
second  and  both  are  quite  membranous.  The  identification  of  two 
sterna  is  furnished  by  the  presence  of  two  spiracles  on  each  side. 


Fig.   18.     "Alluring"  organ  on  abdomen  of  adult  male,  expanded  above, 
retracted  below. 


In  the  female  the  segments  from  two  to  seven  inclusive  are  of 
the  usual  unmodified  type,  but  in  the  male  the  second  segment 
shows  a  peculiar  sexual  dimorphism.  On  the  caudal  margin  of 
the  tergum  of  this  segment  is  located  a  protrusible  organ  which, 
for  want  of  a  better  name,  has  been  termed  an  alluring  gland. 
Similar  organs  called  alluring  glands  have  been  described  as 
occurring  on  other  parts  of  male  Lepidoptera,  and  until  a  histo- 
logical and  cytological  investigation  is  made  of  this  particular  case, 
the  common  term  will  be  used  in  describing  it.  In  other  species 
of  Lepidoptera  there  is  considerable  evidence  that  these  organs 
give  off  a  distinct  odor  when  protruded,  but  the  alluring  function 
of  these  in  a  sexual  sense  is  not  definitely  proved.  This  "gland" 
is  shown  in  figure  18  protruded  (above)  and  retracted  within  the 
abdomen  (below) .  When  retracted  it  folds  in  an  eversible  sac,  and 
when  protruded  the  entire  organ,  including  the  sac,  projects  out 
from  the  body,  looking  for  all  the  world  like  a  composite  flower. 


414 


CONNECTICUT    EXPERIMENT    STATION  BULLETIN    288 


The  scales  composing  it  are  of  two  kinds,  some  pointed  and  some 
lobular.  This  organ  is  found  in  all  males  and  never  in  the 
females.  The  remainder  of  the  male  abdomen  up  to  and  includ- 
ing the  eighth  segment  is  in  no  wise  unusual.  In  the  female  the 
eighth  and  ninth  segments  (figure  19)  are  modified  somewhat. 
On  the  sternum  of  the  eighth  segment  is  a  slight  protuberance 


Fig.  19.     Tip  of  abdomen  of  adult 
female,     bd,  opening  of  bursa  duct. 


Fig.  20.  External  genitalia  of  adult 
male,  ae,  aedoeagus ;  an,  anus ;  lip, 
harpe ;   so,  socius ;  vin,  vinculum. 


which  marks  the  copulatory  opening  leading  into  the  bursa  duct 
and  thence  to  the  bursa  copulatrix.  At  the  end  of  the  ninth  seg- 
ment is  the  external  opening  of  the  vagina  (ventrally)  and  the 
alimentary  tract  (dorsally).  Petersen  (1900)  has  described  in 
some  detail  the  female  and  male  genital  organs  of  Lepidoptera 
and  shows  the  transitional  stages  from  the  type  having  one  genital 
opening  (at  the  tip  of  the  abdomen)  to  that  having  two  as  here 
found.  The  presence  of  two  genital  openings  is  forecast  in  the 
pupa.  On  each  side  of  the  copulatory  opening  is  found  a  tuft  of 
orange-colored  scales,  and  a  third  tuft  is  found  on  the  dorsum  at 
the  anterior  margin  of  the  eighth  segment.  These  three  tufts 
are  normally  concealed  from  view  under  the  posterior  margin  of 
the  seventh  segment.  The  lateral  apodemes  from  the  ninth  pro- 
ject back  into  the  eighth  segment.  The  tip  of  the  female  abdomen 
is  usually  telescoped  so  that  the  eighth  segment  is  partly  retracted 
within  the  seventh,  and  the  ninth  is  retracted  within  the  eighth. 
The  posterior  part  of  the  eighth  is  membranous.  In  the  illustra- 
tion the  abdomen  is  shown  with  these  segments  drawn  out  into 
view.     The  tenth  segment  is  not  developed. 


BIOLOGY    OF    BIRCH    LEAF    SKELETONIZER  415 

In  the  male  (figure  20)  the  ninth  and  tenth  segments  are  much 
modified  and  are  usually  retracted  within  the  eighth.  As  shown 
in  the  figure,  they  are  drawn  out  to  expose  the  external  genitalia. 
The  nomenclature  given  is  according  to  Eyer  (1924).  The  ter- 
gum  of  the  ninth  forms  a  "roof"  over  the  anus  (an)  which  lies 
just  beneath  it.  It  is  called  the  tegumen.  Attached  to  its  distal 
end  are  the  socii  (so)  which  really  belong  to  the  tenth  segment 
and  form  the  anal  armature.  They  are  paired  and  bear  many 
short  spines  and  setae.  The  sternum  of  the  ninth  segment  con- 
sists of  a  narrow  chitinous  band,  the  vinculum  (vin),  which  is 
fused  with  the  tergum  on  each  side.  The  paired  claspers,  called 
harpes  (hp),  articulate  with  the  vinculum  and  are  appendages  of 
the  ninth  segment.  They  also  bear  many  setae  and  short  spines. 
The  cone-like  chitinous  organ  through  which  the  aedoeagus  (ae) 
projects  is  called  the  anellus  and  also  probably  belongs  to  the  ninth 
segment  as  do  the  rest  of  the  genitalia.  The  aedoeagus  is  a  heav- 
ily chitinized  tube  supported  by  the  anellus  and  tapering  to  a  point 
distally.  The  penis  itself  is  a  soft  eversible  tube  contained  within 
the  aedoeagus  and  is  protruded  from  the  ventral  side  of  the  tip  of 
the  latter. 

There  are  on  each  of  the  segments  one  to  seven  inclusive  a  pair 
of  spiracles,  and  visible  through  the  ventral  wall  of  the  abdomen 
are  the  four  pigmented  abdominal  ganglia  of  the  nerve  cord. 
The  ganglia  of  the  entire  ventral  nerve  cord  of  all  stages  of  this 
insect  are  deeply  pigmented  and  usually  visible  externally.  In 
the  adult  the  appendages  conceal  all  but  the  abdominal,  and  these 
are  found  at  the  second  segment  and  at  the  junctions  of  the  third 
and  fourth,  fourth  and  fifth,  and  fifth  and  sixth  respectively. 
The  last  is  larger  than  the  others,  being  a  compound  ganglion. 
The  third  and  fourth  abdominal  ganglia  are  often  contiguous  and 
sometimes  are  fused  to  some  extent. 

6.     Genital  organs  and  alimentary  tract  (Text  figures  21  and  22) 

The  internal  genital  organs  of  the  male  and  female  are  diagram- 
matically  illustrated  in  figures  22  and  21.  In  the  female  the  bursa 
copulatrix  (bur)  is  by  far  the  most  conspicuous  of  these  organs, 
and  it  occupies  much  of  the  anterior  part  of  the  abdomen,  lying 
in  the  region  of  the  third  segment.  It  is  connected  by  a  duct  to 
the  external  opening  in  the  sternum  of  the  eighth  segment,  and 
from  the  dorsal  side  of  this  duct  near  its  external  end  there  arises 
the  long  slender  seminal  duct  which  permits  the  passage  of  sper- 
matozoa from  the  bursa  into  the  oviduct  and  thence  into  the  semi- 
nal receptacle  (rec  sem).  The  common  oviduct  divides  into  two 
ducts  (ovid)  from  each  of  which  are  given  off  four  ovarioles 
(ov),  each  of  which  terminates  in  a  filament.  The  filaments  on 
each  side  unite  with  each  other.     The  ovarioles  extend  from  the 


416 


CONNECTICUT    EXPERIMENT    STATION 


BULLETIN    2.1 


oviduct  along'  each  side  of  the  bursa  to  its  anterior  end,  curve  dor- 
sally  and  posteriorly,  then  dorsally  and  anteriorly  to  a  common 
point  just  above  the  bursa  where  the  group  from  each  side  is 
attached  by  the  filament  tips  to  the  dorsal  wall  of  the  abdomen. 


Fig.  21.  Reproductive  organs  of  female,  bur,  bursa  copulatrix ;  coll  gl, 
colleterial  glands  ;  ov,  ovarioles  ;  ovid,  oviduct ;  rcc  scm,  seminal  receptacle ; 
rcct,  rectum ;    sem  duct,  seminal  duct. 


The  alimentary  tract  passes  ventrally  and  to  the  right  of  the  bursa, 
curves  dorsally  to  pass  above  the  union  of  the  oviducts,  then  goes 
over  the  common  oviduct  to  the  tip  of  the  abdomen,  the  rectum 
lying  above  the  vagina.  The  ovarioles  are  of  the  polytrophic 
type,  that  is,  the  nutritive  cells  alternate  with  the  ova.  The  seminal 
receptacle  is  bilobed  and  is  attached  to  the  dorsal  wall  of  the  com- 
mon  oviduct.     The   colleterial   glands    (coll   gl)    are   paired   and 


BIOLOGY    OF    BIRCH    LEAF    SKELETONlZER 


4'7 


are  connected  by  a  common  duct  to  the  dorsal  wall  of  the  vagina. 
They  secrete  the  adhesive  substance  which  attaches  the  egg  to  the 
leaf.  In  the  illustration  the  genital  organs  are  shown  spread  out 
and  not  in  their  normal  positions. 


/ejac 
duct 


Fig.  22.  Reproductive  organs  of  male,  ac  gl,  accessory  glands ;  aed, 
aedoeagus ;  ejac  duct,  ejaculatory  duct;  test,  testes;  vas  def,  vas  deferens; 
ves  scm,  seminal  vesicle. 


In  the  males  the  testes  (test)  are  united  and  enclosed  in  a  com- 
mon scrotum.  The  vesicula  seminales  (ves  sem)  are  paired  and 
unite  just  under  the  testes.  From  the  vesicula  seminales,  which 
are  really  enlargements  of  the  vasa  deferentia  (vas  def),  the 
latter  ducts  pass  to  enlarged  chambers  which  lead  to  the  ejacu- 
latory duct  (ejac  duct)  which  in  turn  terminates  in  the  aedoeagus 
(ae).  The  accessory  glands  (ac  gl)  which  presumably  secrete 
a  substance  which  mixes  with  the  spermatozoa,  are  paired  and 


418  CONNECTICUT    EXPERIMENT    STATION  BULLETIN    288 

are  connected  with  the  enlargements  at  the  terminations  of  the 
vasa  deferentia.  In  some  insects  these  glands  secrete  a  substance 
which  forms  the  spermatheca  (especially  Orthoptera).  They 
occur  here  attached  to  each  other  rather  loosely. 

The  alimentary  tract  begins  anteriorly  in  a  large  muscular 
pharynx  from  which  two  long  filamentous  salivary  glands  are 
given  off  and  extend  through  the  thorax  beside  the  slender  oesoph- 
agus. The  latter  extends  from  the  pharynx  through  the  thorax 
to  the  mid-gut  in  the  abdomen  and  gives  off  a  slender  tube  which 
enlarges  to  form  a  rather  large  crop.  The  latter  lies  mainly  in 
the  abdomen  over  the  mid-gut.  In  the  female  the  mid-gut  lies 
under  the  bursa  and  extends  to  a  point  between  the  common  ovi- 
duct and  the  bursa.  It  is  much  greater  in  diameter  than  the 
pharynx.  At  the  termination  of  the  mid-gut  the  malpighian  tubes 
are  attached,  one  on  each  side.  These  are  slender  and  each 
branches  once.  Of  the  two  branches  of  each  tube  one  divides 
once  and  the  other  remains  single.  This  gives  three  branches  of 
each  tube  which  lie  in  the  region  of  the  ovarioles.  The  hind-gut 
extends  from  the  origin  of  the  tubules  and  enlarges  at  its  posterior 
end  to  form  the  rectum  which  terminates  in  the  anus.  The  crop 
and  mid-gut  were  empty  in  all  the  specimens  dissected,  which  adds 
evidence  to  the  belief  that  the  adults  do  not  feed.  In  the  male  the 
alimentary  tract  passes  to  the  right  of  the  ejaculatory  duct. 

B.     The  Egg  (Text  figure  23) 

When  first  laid  the  egg  is  a  translucent  white,  but  it  becomes 
more  opaque  after  a  few  days.  In  shape  it  is  flatly  ovoid,  and 
the  longest  diameter  is  0.25  mm.  It  is  made  to  stick  to  the  leaf 
by  an  adhesive  substance  which  surrounds  it  on  the  leaf  surface 
in  a  narrow  encircling  band.  The  hexagonal  sculpturing  of  the 
surface  is  quite  distinct,  and  the  micropyle  occurs  close  to  one  end. 
When  it  leaves  the  oviduct  the  egg  is  soft  and  flexible,  and  its 
shape  changes  from  approximately  spherical  when  it  is  placed  on 
the  leaf  surface.  The  eggs  are  always  laid  singly  and  scattered 
over  the  leaf,  as  shown  on  plate  XVII. 

C.     Larva  (Text  figures  24  and  25) 

When  first  hatched  the  larva  is  minute  (.35  mm.  long),  trans- 
lucent, apodous,  and  flattened,  a  typical  leaf -mining  type.  When 
it  leaves  the  mine  at  the  close  of  the  third  instar  it  has  assumed  a 
cylindrical  form,  the  head  has  shifted  from  its  former  horizontal 
plane  to  a  plane  nearly  at  right  angles  with  the  body,  and  all  the 
legs  are  present  and  functional.  It  measures  about  2.5  mm.  in 
length.  When  fully  grown  (plate  XVII)  the  larva  is  about  6.0 
mm.  long  and  yellowish  green  in  color  with  the  setae  on  white 


BIOLOGY    OF    BIRCH    LEAF    SKELETONIZER 


419 


tubercles.  The  prothoracic  shield  is  not  conspicuous.  The  head  is 
brown  and  typically  that  of  a  leaf -eating-  larva.  There  are  the 
usual  prolegs  on  abdominal  segments,  three,  four,  five,  six  and 
ten. 


.i;'A&?-  '■■•■A; 


Fig.    23.      Egg,    much    enlarged. 
Actual  longest  diameter  .25  mm. 


Fig.  24.  Head  of  larva  of  5th 
instar,  ventral  aspect,  ant,  antenna  ; 
mx,  maxilla ;    oc,  ocellus. 


The  radical  change  in  structure  which  occurs  at  the  second  molt 
is  due  to  the  change  in  feeding  habits  and  environment  of  the 
larva,  for  the  third  instar  comes  out  of  the  mine  to  the  leaf  sur- 
face. This  necessitates  the  acquisition  of  legs  and  the  shifting 
of  the  plane  of  the  head.  The  dorsal  part  of  the  head  capsule  is 
longer  than  the  ventral,  and  this  makes  the  posterior  margin  of 
the  epicranium  in  the  leaf-mining  instars,  where  the  head  is  hori- 
zontal, push  back  into  the  prothorax.  Tragardh  (1913)  has 
described  structural  transitions  in  several  leaf-miners  which 
change  their  feeding  habits.  The  third  instar  resembles  the 
fourth  and  fifth  in  general,  but  the  setal  pattern  is  somewhat 
different  and  there  are  fewer  crochets  on  the  prolegs.  The  pro- 
leg's  of  the  abdominal  segments  three  to  six  inclusive  have  one 
transverse  row  containing  two  crochets,  and  the  prolegs  of  the 
anal  segment  bear  one  crochet. 

The  fourth  instar  is  like  the  fifth  except  for  size  and  no  further 
mention  need  be  made  of  it.  The  fifth  instar  is  a  typical  cater- 
pillar with  the  mouth-parts  well  developed.  These  are  shown  in 
a  ventral  view  in  figure  24.  The  antennae  are  minute,  and  there 
are  only  five  ocelli  (oc)  present  on  each  side.     They  are  arranged 


420 


CONNECTICUT    EXPERIMENT    STATION 


BULLETIN  -21 


A 

\ 

(;, 

N. 

q_        *= 

\ 

»«i 

fe 


< 


BIOLOGY    OF    BIRCH    LEAF    SKELETONIZER  42  I 

in  a  curved  row  whose  concavity  is  ventral.  The  labium  is  drawn 
out  into  a  spinneret  through  which  the  duct  of  the  silk  glands 
reaches  the  exterior.  The  maxillae  bear  on  the  inner  surface  a 
pair  of  curved  chitinous  hooks.  Otherwise  the  mouth-parts  are 
not  unusual.  On  the  dorsal  surface  the  adfrontals  extend  back 
to  the  posterior  margin  of  the  epicranium  and  the  frons  is  about 
one-third  the  length  of  the  head  capsule.  The  labrum  is  bilobed 
and  the  mandibles  bear  four   "teeth." 

The  prothoracic  legs  differ  from  those  of  the  meso-  and  meta- 
thorax  in  that  the  terminal  segment  bears  one  claw  only,  whereas 
in  the  case  of  the  latter  two  pairs  of  legs  the  claw  is  protected 
by  a  pair  of  terminal  lappets.  The  prolegs  of  the  abdominal  seg- 
ments three  to  six  inclusive  bear  on  the  planta  two  rows  of 
crochets,  three  per  row.  The  anal  prolegs  have  a  single  crochet 
only. 

The  setal  pattern  of  the  body  of  the  larva  is  of  some  taxonomic 
importance  and  is  shown  in  figure  25.  In  this  description  the 
nomenclature  of  Fracker,  though  cumbersome,  has  been  followed. 
The  diagrams  are  made  so  that  the  anterior  margin  of  each  seg- 
ment is  to  the  left  and  the  dorsal  midline  is  at  the  top.  Each  dia- 
gram is  that  of  the  left  half  of  each  segment  projected  on  a  flat 
surface.  Certain  segments  are  alike,  and  these  have  been  repre- 
sented by  one  diagram.  The  setae  of  the  anal  segment <  do  not 
conform  to  those  of  any  of  the  others. 

D.     Pupa  (Text  figures  26  to  29) 

The  pupa  is  spindle-shaped,  about  three  millimeters  long  and 
brown  in  color.  Many  of  the  adult  structures  are  evident,  and 
the  head  thorax  and  abdomen  are  distinct.  In  the  description 
here  given  the  parts,  although  often  incompletely  developed,  are 
named  in  accordance  with  the  corresponding  parts  of  the  adult. 

The  vertex  (vert)  occupies  most  of  the  dorsal  side  of  the  head 
and  is  separated  from  the  frons  by  the  Y-shaped  epicranial  suture, 
the  frons  meeting  the  arms  of  the  suture.  The  stem  of  the  Y  is 
indistinct.  The  frons  (fr)  extends  caudally  along  the  ventral 
side  of  the  head  from  the  epicranial  suture  to  merge  into  the 
clypeal  region,  there  being  no  demarcation  between  the  two.  The 
frons  in  figure  26  can  be  easily  distinguished  by  the  presence  of 
the  pointed  cutting  plate  in  its  anterior  part.  This  is  the  so-called 
"cocoon-breaker"  with  the  aid  of  which  the  pupa  emerges  from 
the  cocoon.  The  bases  of  the  antennae  (ant)  are  visible  on  the 
dorsal  side  of  the  head  lateral  to  the  epicranium.  On  the  ventral 
side  of  the  head  and  lateral  to  the  frons  are  the  eye-pieces  (e). 
The  clypeus  bears  the  bilobed  labrum,  and  on  each  side  of  the 
labrum  is  a  small  triangular  mandibular  sclerite.  Neither  labrum 
nor  mandibles  are  found  in  the  adult.     The  labrum  bears  a  pair 


422 


CONNECTICUT    EXPERIMENT    STATION 


BULLETIN    21 


of  laterally  placed  setae.  The  maxillae  are  prominent  and  form 
the  pair  of  medially  placed  appendages  extending-  caudally  from 
the  lab  rum.  Neither  maxillary  nor  labial  palpi  are  visible.  The 
antennae  extend  caudally  almost  to  the  tips  of  the  wings.     Be- 


Fig.  26.  Female  pupa,  ventral 
aspect,  much  enlarged,  ant,  an- 
tenna ;  cl,  clypeus ;  ex,  coxa ;  e, 
eye ;  fr,  f  rons  ;  max,  maxilla ;  sp, 
spiracle. 


Fig.    2J.      Pupa,    dorsal    aspect, 
much  enlarged. 


tween  the  antennae  and  maxillae  lie  the  folded  prothoracic  and 
mesothoracic  legs.  The  tarsal  regions  of  the  metathoracic  legs 
are  visible  between  the  tips  of  the  antennae,  most  of  this  pair  of 
appendages  being  covered  by  those  preceding.  A  small  part  of 
the  metathoracic  coxae  is  visible  in  the  midline  posterior  to  the 
mesothoracic  coxae.  The  metathoracic  legs  extend  slightly  be- 
yond the  tips  of  the  wings.  The  fore  wings  extend  to  the  seventh 
segment  of  the  abdomen  on  the  ventral  side  and  conceal  the 
hind  wings.     The  appendages  are  loosely  attached  to  each  other 


BIOLOGY    OF    BIRCH    LEAF    SKELETONIZER  423 

and  are  free  from  the  body  wall.  They  overlap,  more  or  less,  and 
the  covered  parts  are  quite  membranous.  When  dissected  out, 
the  regions  of  the  coxa,  femur,  tibia  and  tarsus  are  visible, 
although  often  not  distinctly  demarcated,  and  the  tibial  spurs  are 
prominent.  The  two  folds  in  the  legs  occur  between  coxa  and 
femur  and  between  femur  and  tibia.  The  tibia  merges  into  the 
tarsus. 


Fig.  28.     Tip  of  abdomen  of  male  Fig.   20.     Section   of   dorsum   of 

pupa,  ventral  aspect.  abdomen  of  pupa. 


The  female  has  evidence  of  two  genital  openings  on  the  ventral 
side,  one  on  the  posterior  border  of  the  eighth  abdominal  segment 
and  one  on  the  interior  border  of  the  ninth.  These  are  short 
slits  in  the  integument.  The  male  has  evidence  of  one  genital 
opening  only  (figure  28),  on  the  ninth  segment.  This  condition 
in  each  sex  corresponds  to  that  of  the  adult.  On  the  tip  of  the 
tenth  abdominal  segment  is  the  indentation  marking  the  anal  open- 
ing, and  on  the  lateral  sides  of  this  segment  are  a  pair  of  short 
strong  spines. 

On  the  dorsal  side  of  the  pupa  the  prothorax  is  constricted  in  the 
middle  and  widens  laterally.  It  lies  between  the  epicranium  and 
mesothorax  and  abuts  on  the  antennae.  The  epicranial  suture  ex- 
tends to  the  anterior  margin  of  the  prothorax.  The  mesothorax  is 
a  large  quadrate  sclerite  separated  by  distinct  sutures  from  the  pro- 
thorax, metathorax,  and  fore  wings.  Along  its  midline  it  is  raised 
into  a  very  slight  ridge.  At  the  anterior-lateral  angles  are  a  pair 
of  setae.  The  mesothoracic  wings  extend  around  the  body  to  the 
ventral  side.  The  metathorax  is  not  so  long  as  the  mesothorax 
and  merges  indistinctly  into  the  wings  laterally.  It  bears  also  a 
pair  of  setae  at  the  anterior-lateral  angles,  but  these  are  more 
approximated  than  those  of  the  mesothorax.  The  metathoracic 
wings  are  almost  entirely  concealed  by  those  of  the  mesothorax, 
the  bases  only  being  visible.  There  are  visible  dorsally  ten 
abdominal  segments,  of  which  numbers  two  to  seven  inclusive 
bear  a  pair  of  setae  at  the  anterior-lateral  angles,  and  numbers 
one  to  seven  bear  laterally  placed  spiracles.  Segments  four  to 
seven  inclusive  bear  also  a  pair  of  medially  placed  setae.  The 
spiracles  on  the  first  abdominal  segment  are  concealed  by  the  hind 


424  CONNECTICUT    EXPERIMENT    STATION  BULLETIN    288 

wing's.  At  the  anterior  margin  of  the  tenth  segment  is  a  dorsal 
tubercle  bearing  a  pair  of  spines.  This  and  the  lateral  spines  on 
the  tenth  segment  are  purely  pupal  structures.  The  dorsal  sur- 
face of  the  abdominal  segments  is  covered  with  minute  spines 
(figure  29)  and  on  segments  two  to  seven  inclusive  there  is  a  row 
of  heavy  spines  along  the  anterior  margin. 

In  the  male  abdominal  segments  three  to  seven  are  movable,  and 
in  the  female  segments  three  to  six  are  movable.  The  terminal 
segments  are  immovably  united  in  both  sexes. 

VI.     Life  History  and  Habits 

All  the  data  here  given,  except  for  field  records  and  other  cases 
specifically  mentioned,  were  obtained  from  records  of  individual 
insects  reared  in  New  Haven,  Conn.,  on  the  gray  birch,  Betula 
populifolia.  The  field  observations  in  Connecticut  and  Massa- 
chusetts are  also  of  insects  occurring  on  Betula  populifolia  unless 
otherwise  stated.  The  period  during  which  records  were  made 
covers  the  years  1924,  1925,  and  1926. 

The  first  adults  appear  the  last  of  June  in  the  region  about  New 
Haven,  and  the  last  disappear  the  last  part  of  July.  In  1924 
adults  were  fairly  numerous  July  7th,  and  the  last  were  seen  July 
31st.  During  1926  adults  were  systematically  collected  with  a  net 
and  by  hand  during  July  in  one  locality,  a  group  of  birches  just 
north  of  Mt.  Carmel,  near  New  Haven,  and  these  collections' 
indicated  a  maximum  number  of  adujts  were  present  the  fourth 
week  in  July.  During'  1926  the  season  was  later  than  usual. 
This  species  was  abundant  during  the  second  and  third  weeks  in 
July,  but  from  the  27th  to  the  31st  it  declined  in  numbers  from 
about  a  maximum  to  disappearance. 

Pupae  were  kept  at  normal  temperatures  in  an  out-door 
screened  insectary  during  1924,  1925,  and  1926.  Those  of  1924 
were  collected  in  the  field  during  the  spring  of  that  year;  those 
of  1925  and  1926  were  reared  in  the  out-door  insectary.  Records 
were  kept  of  the  emergence  of  104  adults  in  1924,  135  in  1925, 
and  36  in  1926.  In  1924  the  period  of  emergence  was  between 
June  4th  and  July  9th  (only  three  emerged  before  June  23d)  ;  in 
1925  between  June  15th  and  July  19th,  and  in  1926  between  July' 
2d  and  July  21st.     The  period  of  maximum  emergence  during 

1924  was  between  June  25th  and  July  9th ;  in  1925  between  June 
1 8th  and  July  10th,  and  in  1926  between  July  2d  and  July  9th.    In 

1925  all  records  obtained  after  July  1st  were  of  individuals  taken 
from  New  Haven  to  Woods  Hole,  Mass.  The  early  appearance 
of  adults  in  1925  may  possibly  have  been  due  to  high  temperatures 
early  in  June  of  that  year,  for,  during  the  first  ten  days  of  June, 
1925,  the  mean  hourly  temperature  was  75.2°F.  as  opposed  to 
6o.6°F.  in  1926,  and  6i.2°F.  in  1924  (U.  S.  Weather  Bureau, 
New  Haven,  Conn.,  statistics). 


BIOLOGY    OF    BIRCH    LEAF    SKELETONIZER  425 

Hutchings  (1926)  reports  that  in  Ontario  the  adults  are  found 
during-  July  (up  to  the  25th)  with  a  maximum  emergence  from 
July  6th  to  July  14th.  In  New  Brunswick,  Gorham  (1922) 
reports  adults  abundant  the  first  two  weeks  of  July.  It  would 
seem  from  these  reports  that  the  adult  insect  appears  at  approxi- 
mately the  same  time  of  year  over  much  of  its  entire  range,  being 
at  a  maximum  during  the  first  part  of  July. 


Fig.  30.     Cast  pupal  skin  and  cocoon. 

From  the  pupae  collected  in  the  field  in  the  spring  of  1926  there 
were  secured  54  adults  of  which  the  sexes  were  determined. 
Thirty-three  were  females  and  21  were  males.  Of  36  adults 
secured  from  laboratory-reared  larvae,  19  were  females  and  17 
were  males.  Although  the  females  outnumber  the  males,  the  dif- 
ference is  not  sufficiently  great  to  warrant  an  assumption  that 
there  is  not  approximately  an  equal  number  of  each  sex  under 
natural  conditions. 

When  ready  to  emerge  from  the  cocoon  the  insect  is  in  the  pre- 
imaginal  stage,  and  the  scales  of  the  imago  are  easily  seen  through 
the  pupal  skin.  The  pre-imago  works  its  way  forward,  probably 
with  the  assistance  of  the  spines  on  the  tenth  abdominal  segment, 
and  breaks  through  the  anterior  end  of  the  cocoon.  The  cutting 
plate  on  the  vertex  is  of  material  aid  in  the  process.  When  about 
three-fourths  of  the  length  is  exposed,  the  body  is  held  at  an  angle 
of  about  400  from  the  cocoon.  The  pupal  skin  splits  at  the  junc- 
tion of  the  vertex  and  the  prothorax  and  also  longitudinally 
through  the  prothorax  and  mesothorax.  The  eye-pieces  remain 
attached  to  the  antennae  and  mouth  parts.  Figure  30  shows  a 
cocoon  and  an  empty  pupal  skin  in  the  characteristic  position. 


425  CONNECTICUT    EXPERIMENT    STATION  BULLETIN    288 

According  to  Snodgrass  (1922),  the  emergence  from  the  cocoon 
is  greatly  assisted  in  Bucculatrix  pomifoliella  Clemens  by  the 
presence  of  three  "valves"  in  the  anterior  end  of  the  cocoon, 
these  acting  as  an  inclined  plane  to  lift  the  insect.  In  B.'cana- 
densisella  these  structures  are  not  present.  The  inner  part  of  the 
cocoon  is  woven  so  that  its  lateral  walls  curve  in  at  the  base,  mak- 
ing a  "round  corner,"  and  this  might  conceivably  give  the  pre- 
imago  an  upward  lift.  However,  if  a  pupa  is  pushed  gently 
forward  by  hand,  it  does  not  emerge  at  the  normal  place,  but 
lower  down  near  the  base  of  the  cocoon.  By  simply  rolling  over 
in  its  attempt  to  extricate  itself,  the  pre-imago  would  bring  the 
cutting  plate  in  a  more  dorsal  position  with  respect  to  the  cocoon 
and  thus  insure  emergence  at  the  normal  location. 

During  the  daylight  hours  the  adults  rest  quietly  on  the  surfaces 
of  the  lower  leaves  of  birches,  or  of  the  leaves  of  undergrowth 
under  the  birches,  or  on  the  lower  part  of  the  trunks  of  the  trees. 
They  fly  very  little  unless  disturbed,  and  usually  remain  motionless 
for  long  periods  of  time.  The  photograph  on  plate  XVII  is  a  three- 
minute  exposure  of  a  live  adult  which  was  carried  to  and  from 
the  camera  in  the  position  seen.  It  shows  the  adult  in  the  char- 
acteristic position.  They  can  be  collected  very  easily  by  simply 
inverting  an  empty  glass  vial  over.- the  quiet  insect  and  then  gently 
tapping  the  opposite  side  of  the  leaf  on  which  it  rests.  It  will 
quickly  fly  or  run  up  into  the  vial.  I  have  collected  one  hundred 
in  less  than  an  hour  from  fern  fronds  in  this  manner.  When  dis- 
turbed, they  fly  quickly  a  few  feet,  very  rarely  over  five  or  six, 
and  usually  much  less.  If  confined  in  a  bottle  they  run  excitedly 
for  a  few  minutes  when  disturbed. 

From  field  observations  it  seems  that  as  a  rule  the  moths  remain 
near  the  ground  during  the  day  and  go  up  into  the  trees  at  about 
dusk.  I  have  often  failed  to  get  them  in  a  net  by  sweeping  birches 
over  my  head,  when  many  were  secured  by  sweeping  within  four 
feet  of  the  ground.  Yet,  in  the  same  location,  by  sweeping  the 
birches  at  dusk,  several  were  netted  very  quickly  at  a  height  of 
about  nine  feet  from  the  ground.  In  1926  in  one  particular 
locality,  moths  were  very  abundant  during  the  day  on  the  fronds 
of  ferns  growing  under  the  birch  trees,  but  none  were  seen  on  the 
leaves  of  the  birches  which  were  not  lower  than  four  feet  from 
the  ground.  An  investigation  of  the  birches  at  night  with  the 
aid  of  an  acetylene  lamp  showed  that  the  moths  were  all  up  on 
the  birch  leaves,  and  none  were  on  the  ferns  below.  The  next 
day  over  one  hundred  were  easily  caught  on  the  ferns,  but  none 
were  on  the  leaves  of  the  trees.  At  times,  however,  I  have  found 
moths  on  the  leaves  of  the  trees  during  the  day  five  feet  from  the 
ground.  Gorham  (1922)  reports  that  in  New  Brunswick  he 
found  moths  on  the  birches  in  large  numbers  at  all  hours  of  the 
day,   but   he  gives   no    further  information   on   the   distribution. 


BIOLOGY    OF    BIRCH    LEAF    SKELETONIZER  427 

The  moths  prefer  situations  out  of  direct  sunlight,  and  this  may 
account  for  their  position  during  daylight  hours.  This  nightly 
migration  into  the  trees  may  be  affected  somewhat  by  conditions 
of  light,  moisture,  and  amount  of  undergrowth,  but  it  apparently 
occurs  to  some  extent  wherever  conditions  are  normal. 

The  response  of  these  moths  to  light  is  not  very  definite,  due  to 
their  habit  of  remaining  quiescent  in  one  spot  unless  greatly  dis- 
turbed. Attempts  to  make  them  show  either  a  negative  or  posi- 
tive response  to  daylight  in  the  laboratory  gave  inconclusive 
results.  Several  attempts  to  attract  them  to  a  lighted  lantern,  an 
acetylene  lamp,  and  automobile  headlights  at  night  in  the  field 
failed  completely,  although  they  were  present  in  considerable 
numbers  on  the  surrounding  trees  and  were  relatively  more  active 
than  during  daylight. 

From  the  above  observations  it  is  assumed  that  oviposition 
occurs  at  night.  In  the  insectary  eggs  Avere  secured  from  three 
to  seven  days  after  the  adults  emerged,  but  in  view  of  the  fact  that 
the  moths  are  very  inconsistent  about  ovipositing"  in  captivity, 
these  data  may  have  to  be  extended  under  normal  conditions. 

It  was  necessary  to  place  several  males  and  females  in  a  cage 
in  order  to  ensure  a  supply  of  eggs.  Although  I  have  no  data  on 
the  number  of  eggs  laid  by  any  one  female,  an  examination  of  the 
oviducts  shows  that  there  may  be  a  considerable  number,  for  sixty- 
two  fully  formed  eggs  were  dissected  out  from  one  female  caught 
in.  the  field,  and  there  may  have  been  several  laid  before  she  was 
captured.  This  number  of  eggs  was  never  secured  from  any 
female  in  captivity.  From  four  females  thirty-four  eggs  were 
secured  in  one  day  in  a  cage,  and  from  these  same  four,  fourteen 
eggs  the  following  day.  They  died  without  further  oviposition. 
These  females  were  reared  and  laid  only  the  eggs  recorded. 

The  adults  have  been  kept  alive  in  cages  out  of  doors  for  twelve 
days  after  emergence  from  the  cocoon,  but  they  usually  die  sooner. 
They  have  never  been  seen  to  feed,  and  apparently  they  did  not 
touch  a  honey-and-water  mixture  placed  in  the  cage.  The  pres- 
ence of  a  25  per  cent  solution  of  honey  in  the  cage  did  not  prolong 
the  duration  of  adult  life.  Certainly  food  is  not  a  requisite  and 
is  not  necessary  to  oviposition.  All  the  adults  collected  in  the 
field  died  in  a  few  days,  so  twelve  days  probably  is  a  fairly  long 
period  of  life.  Humidity  and  temperature  have  considerable 
effect  on  this,  and  moths  caught  in  the  field  can  be  kept  alive  four 
to  nine  days  if  held  at  io°-i2°C.,  whereas  they  die  in  one  to  three 
days  at  room  temperature.  They  will  remain  active  after  an 
exposure  to  7°C.  for  twelve  hours,  but  when  the  air  is  cooled  to 
5°C.  they  very  quickly  become  inactive. 

The  eggs  are  laid  singly  on  either  side  of  the  leaf  and  on  any 
part  of  the  surface.  There  is  some  preference  shown  for  a  posi- 
tion beside  the  midrib  or  some  other  prominent  vein  of  the  leaf. 


428  CONNECTICUT    EXPERIMENT    STATION  BULLETIN    288 

but  not  to  the  exclusion  of  the  rest  of  the  leaf  (plate  XVII).  Eggs 
are  laid  on  leaves  on  all  parts  of  the  gray  birch.  Insectary  records 
for  1924  showed  a  period  of  oviposition  lasting  from  July  5th  to 
July  21st,  and  field  observations  the  same  year  showed  unhatched 
eggs  up  to  August  7th.  No  field  observations  were  made  dur- 
ing 1925,  but  gray  birches  sent  from  Bostony  Mass.,  to  Woods 
Holt,  Mass.,  on  July  9th,  carried  many  unhatched  eggs  on  the 
leaves.  In  1926  eggs  were  found  in  the  field  between  July  23d 
and  August  3d,  but  in  view  of  the  fact  that  they  were  numerous 
July  23d  and  that  some  of  those  collected  that  day  hatched  July 
30th,  oviposition  must  have  begun  as  early  as  July  16th.  Many 
eggs  collected  August  3d  hatched  August  15th,  so  oviposition 
occurred  as  late  as  August  1st.  Oviposition  takes  place  usually 
during  the  month  of  July,  and  unhatched  eggs  may  be  found  up 
to  the  middle  of  August.  In  1926  the  incubation  periods  of  48 
eggs  from  laboratory-reared  adults  were  15  days  on  the  average, 
the  maximum  being  17  days,  the  minimum  13  days,  and  the 
majority  (27)  taking  14  days.  In  all  but  three  cases  these  eggs 
went  through  the  incubation  period  between  June  25th  and  July 
13th,  a  month  earlier  than  normal.  They  were  from  laboratory- 
reared  adults  which  emerged  earlier  than  normal.  The  other 
three  eggs  were  incubated  at  the  normal  time,  between  July  31st 
and  August  14th,  and  they  took  14  days  each,  so  the  figure  for 
all  48  was  normal.  This  agrees  with  the  period  given  by  Hutch- 
ings  in  Ontario  in  1925.  The  eggs  have  a  high  degree  of  fer- 
tility, and  those  that  do  not  hatch  are  rare. 

A  short  time  before  the  larva  leaves  the  egg  it  can  be  seen  curled 
inside  (figure  31).  When  it  emerges,  the  young  larva  bores 
through  the  bottom  of  the  egg  into  the  leaf,  and  as  it  feeds  it 
leaves  the  egg  filled  with  dark  excrement.  This  habit  makes  it 
very  easy  to  determine  whether  or  not  the  eggs  are  hatched,  for 
after  the  larva  has  left,  the  egg  appears  brown  or  black  in  contrast 
to  its  former  translucent  condition.  For  several  days  the  young 
larva  mines  close  to  the  egg,  but  it  finally  straightens  its  path  and 
mines  in  a  more  or  less  definite  direction. 

The  larva  completes  the  first  and  second  instars  and  most  of 
the  third  in  the  mine.  I  have  found  a  head  capsule  in  a  mine  only 
twice,  but  the  measurements  of  the  width  of  the  head  capsules 
(p.  444)  and  the  descriptions  of  the  larvae  clearly  indicate  three 
mining  instars.  While  in  the  mine  the  larva  is  always  oriented 
dorso-ventrally  with  the  leaf ;  that  is,  its  dorsal  side  is  always 
toward  the  upper  surface  of  the  leaf.  For  the  first  week  the  mine 
is  extended  very  little  and  is  always  close  to  the  egg,  giving  a 
blotch  appearance,  due  to  continuous  turning  of  the  mine  in  a 
small  area.  A  mine  six  days  old  measured  only  1.5  mm.  across 
the  mined  area.  This  larva  never  makes  a  real  blotch  mine,  but 
its    excavations   are   always    linear    and    winding,    with    slightly 


BIOLOGY    OF    BIRCH    LEAF    SKELETONIZER 


429 


Fig.  31.  Larval  mine  in  birch  leaf,  slightly  less  than  normal  size  (upper 
left)  ;  embryo  in  egg  (upper  right)  ;  larva  in  molting  web  (lower  left)  ; 
diagram  of  cross  section  of  cocoon   (lower  right). 


enlarged  ends.  During  the  last  part  of  its  mining  life  the  larva 
lengthens  the  mine  very  rapidly  and  broadens  it  somewhat.  Most 
of  the  mines  are  about  three-fourths  of  an  inch  long  when  finished 
(figure  31). 

When  ready  to  emerge  from  the  mine,  the  larva  cuts  a  crescentic 
opening  in  the  lower  epidermis,  an  operation  taking  about  fifteen 
minutes.  It  then  works  its  way  out  until,  by  bending  its  body 
ventrally,  it  can  grip  the  leaf  surface  with  its  thoracic  feet.  It 
then  quickly  pulls  itself  out  of  the  mine,  the  entire  performance 
consuming  about  two  and  one-fourth  minutes.     While  emerging 


43°  CONNECTICUT    EXPERIMENT    STATION  BULLETIN    288 

from  the  mine,  the  larva  spins  a  few  long-  threads  of  silk  in  the 
shape  of  a  figure  "8."  I  was  unable  to  detect  any  use  to  which 
this  peculiar  habit  could  be  put.  The  opening  through  which  the 
larva  emerges  is  always  crescentic,  always  of  the  same  approxi- 
mate size,  and  always  cut  in  the  lower  surface  of  the  leaf  regard- 
less of  which  surface  bears  the  egg.  Hundreds  of  mines  have 
been  examined,  and  these  were  made  by  larvae  which  hatched 
from  eggs  laid  on  either  side  of  the  leaf,  and  only  once  an  exit 
was  found  on  the  upper  surface. 

It  was  thought  that  both  light  and  gravity  might  cause  the  larva 
to  orient  itself  dorso-ventrally  with  the  leaf,  and  if  this  were  true, 
the  larva  might  be  made  to  change  its  orientation  by  either  invert- 
ing the  leaf  or  changing  the  incidence  of  light  on  it.  The  upper 
surface  of  a  leaf  containing  six  mining  larvae  was  covered  over 
with  black  paper,  and  a  mirror  was  so  placed  that  daylight  was 
reflected  on  the  lower  surface.  This  caused  light  to  fall  on  the 
ventral  surface  of  the  larvae  instead  of  on  the  dorsal,  as  under 
normal  conditions.  After  an  interval  of  15  days  all  the  larvae 
had  emerged  from  the  lower  surface  of  the  leaf.  The  change  in 
incidence  of  light  had  no  effect.  A  branch  of  a  gray  birch  bear- 
ing several  leaves  which  contained  mining  larvae  was  inverted  so 
that  the  morphologically  lower  surface  was  uppermost.  This 
changed  the  relation  of  the  larvae  to  gravity.  Under  natural  con- 
ditions the  leaves  never  hang  horizontally,  so  the  orientation  of 
the  larvae  is  only  relative  at  best.  The  leaves  were  examined 
after  an  interval  of  18  days.  Approximately  50  larvae  had 
emerged  from  the  mines,  and  of  these  only  two  had  emerged 
through  the  morphologically  upper,  now  lower,  surface.  In  view 
of  the  fact  that  occasionally  a  larva  emerges  from  the  upper  sur- 
face normally,  the  emergence  of  these  two  is  of  no  significance. 
It  is  quite  evident  that  changing  the  orientation  to  either  light  or 
gravity  has  no  effect  on  the  position  of  the  insect  relative  to  the 
morphologically  upper  and  lower  surfaces  of  the  leaf  after  the 
mine  is  well  under  way ;  nor  will  the  surface  of  the  leaf  through 
which  the  larva  emerges  from  the  mine  be  changed  by  any  such 
procedure.  In  the  above  cases  all  the  mines  were  about  half 
finished  when  the  conditions  were  changed.  The  orientation  of 
the  larva  must  be  determined  after  it  bores  through  the  epidermis 
of  the  leaf  from  the  egg  and  before  it  mines  to  any  great  extent, 
for  all  larvae  examined  were  found  with  the  ventral  side  toward 
the  lower  side  of  the  leaf  even  when  the  mine  was  only  two  or 
three  days  old. 

Where  the  insect  is  abundant  it  is  by  no  means  unusual  to  find 
25  to  40  mines  in  one  leaf.  The  mine  shows  more  clearly  through 
the  upper  epidermis  of  the  leaf  than  through  the  lower,  but  this 
may  be  due  to  differences  in  the  structure  of  these  parts  of  the 
leaf  rather  than  to  the  nature  of  the  mine.     The  larvae  are  disin- 


BIOLOGY    OF    BIRCH    LEAF    SKELETONIZER  43 1 

clined  to  gnaw  through  large  veins,  and  usually  the  mine  turns 
aside  at  these  obstructions. 

The  duration  of  the  mining  stage  varies  greatly.  The  maxi- 
mum period  observed  was  50  days,  the  minimum  13  days.  This 
variation  may  be  due  partly  to  the  conditions  under  which  the 
larvae  were  reared.  In  1924  and  1925  all  the  larvae  were  reared 
at  the  normal  season  and  fed  on  leaves  in  normal  condition.  Dur- 
ing these  two  years  the  maximum  duration  of  larval  life  was  37 
days,  and  the  minimum  24  days.  In  1926  several  larvae  were 
reared  from  eggs  laid  by  laboratory-reared  adults  a  month  earlier 
than  normal — in  June,  in  fact — and  the  foliage  of  the  birches  was 
in  a  more  rapidly  growing"  and  tender  condition,  especially  since 
these  birches  were  more  or  less  sheltered  and  were  well  fertilized 
and  watered.  The  water  content  of  the  leaves  must  have  been 
greater  than  larvae  would  normally  meet.  All  of  these  larvae 
but  two1  were  in  the  mines  over  32  clays,  and  the  average  period 
of  20  was  44  days.  Of  the  two  remaining,  one  was  in  the  mine 
16  days  and  the  other  13  days.  These  22  individuals  are  not 
included  in  computing  the  mining  period.  All  of  the  other  larvae 
reared  during  1926,  35  in  number,  on  which  records  were  kept  of 
the  mining  period,  were  in  the  mines  30  days  or  less.  The  dura- 
tion of  the  mining  stage  during  1924,  1925,  and  1926  was  on  the 
average  22  to  27  days  for  50  individuals,  not  including  the  22 
mentioned  above.  This  figure  is  probably  correct  for  normal  con- 
ditions. Hutchings  (1926)  gives  the  mining  period  as  seven  to 
eight  days  in  Ontario,  but  such  a  short  mining  period  would  bring 
the  larvae  to  a  fully  grown  condition  much  earlier  than  they  really 
appear  in  the  field.  There  is  a  difference  in  food  plants  to  be 
considered,  for  larvae  in  New  Haven  were  reared  on  gray  birch, 
whereas  the  common  birches  in  Ontario  attacked  by  this  insect  are 
the  yellow  (B.  lutea)  and  white  (B.  papyrifera).  Nevertheless  in 
Connecticut  the  larvae  appear  feeding  externally  on  gray  and 
white  birches  at  the  same  time,  which  indicates  a  similar  mining 
period. 

Mining  larvae  have  been  observed  in  the  woods  about  New 
Haven  as  early  as  August  6th,  and  eggs  collected  on  leaves  have 
hatched  July  30th  when  brought  into  the  laboratory  a  week  earlier. 
Larvae  have  been  found  out  of  the  mines  August  6th,  but  this  is 
unusually  early.  The  period  when  mining  larvae  occur  around 
New  Haven  lies  approximately  between  August  1st  and  Septem- 
ber 15th,  with  a  maximum  number  present  the  fourth  week  in 
August.     The  rearing  records  coincide  with  these  limits. 

When  once  free  from  the  mine,  the  larva  wanders  over  the  leaf 
for  a  short  time,  an  hour  or  two,  and  then  spins  its  first  molting 
web.  A  larva  has  never  been  seen  to  feed  between  the  emergence 
from  the  mine  and  the  spinning  of  the  web.  There  may  be  a 
difference  in  different  species  of  the  genus,  for  Chambers  (1882) 


432  CONNECTICUT    EXPERIMENT    STATION  BULLETIN    2S8 

observed  that  Bucculatrix  ambrosiaefoliella  feeds  two  days 
between  emerging  from  the  mine  and  molting-.  The  larva  which 
emerges  from  the  mine  is  structurally  more  like  the  following 
external  feeding  instar  than  the  preceding  mining  instar.  This 
may  argue  for  possible  external  feeding,  but  there  is  no  evidence 
that  it  occurs  at  this  time.  The  exact  interval  of  time  between 
emergence  from  the  mine  and  either  beginning  or  completion  of 
the  first  molting  web  was  determined  for  three  larvae,  all  typical 
cases.  One  larva  emerged  from  the  mine  at  8.20  a.  m.  and  com- 
pleted its  web  at  10.45  A-  M-  >  one  emerged  from  the  mine  at  9.40 
a.  m.,  began  its  web  at  10.40  a.  m.,  and  completed  it  at  12.10  p.  m.; 
one  emerged  from  the  mine  at  10.09  A-  M->  began  its  web  about 
10.42  a.  m.,  and  finished  it  at  12.12  p.  m.  The  larva  often  selects 
a  position  beside  a  large  vein  for  its  web,  but  it  will  also  spin  on 
the  flat  upper  surface  of  the  leaf.  There  seems  to  be  a  preference 
for  a  hollow  over  which  the  "roof"  of  the  web  may  be  spun,  as 
the  angle  between  the  base  and  sides  of  a  glass  bottle  or  the  hol- 
low beside  the  midrib  of  the  leaf.  Having  selected  a  suitable 
location,  the  larva  lays  down  a  thin  basal  "floor"  web  on  the  sur- 
face of  the  leaf.  This  is  about  1.5  mm.  in  diameter.  Then  it 
spins  another  web  over  this,  making  long  tacks  from  side  to  side 
by  swinging  the  entire  thorax  and  the  first  two  abdominal  seg- 
ments from  one  side  to  the  other.  The  body  is  held  facing  out 
and  the  threads  are  always  straight.  In  shifting  its  position  the 
larva  swings  the  abdomen  quickly  almost  180  degrees.  The  periph- 
ery of  the  web  is  thus  built  up  first,  the  center  being  weak.  A 
series  of  short  tacks  is  now  made  over  the  "frame"  of  long 
threads,  and  the  center  is  strengthened.  This  is  followed  by  a 
series  of  short  tacks  all  around  the  edge,  a  proceeding  which  evi- 
dently strengthens  the  web.  A  hole  is  quickly  made  through  the 
web  near  the  center,  and  the  larva  crawls  in  head  first  between  the 
"floor"  and  the  "roof."  In  crawling  into  its  molting  chamber 
the  larva  doubles  ventrally  so  that  its  back  is  down  on  the  "floor" 
and  its  feet  touch  the  "roof";  that  is,  it  is  oriented  dorsally  to 
the  leaf.  Before  it  is  all  inside,  the  larva  swings  its  head  to  and 
fro,  weaving  a  mat  on  the  under  side  of  the  "roof."  Since  the 
diameter  of  the  web  is  not  much  more  than  half  the  length  of  the 
larva,  the  latter  is  forced  to  turn  around,  and  when  completely 
inside,  its  head  almost  touches  the  last  abdominal  segment,  the 
body  being  bent  in  a  U  shape  and  to  the  right  or  left  (figure  31). 
It  is  plainly  visible  through  the  web.  The  larva  is  not  content 
with  getting  inside,  but  actually  makes  a  turn  around  its  molting 
chamber.  All  this  time  it  swings  its  head,  weaving  figure  "8" 
loops,  and  in  due  time  it  incidentally  has  to  cover  the  hole  in  the 
"roof"  by  which  it  entered.  Although  this  opening  is  always  cov- 
ered, the  larva  seems  to  make  no  deliberate  attempt  to  cover  it, 
doing  so  eventually  as  it  works  around  inside.     Most  of  the  weav- 


BIOLOGY    OF    BIRCH    LEAF    SKELETONIZER  433 

ing"  is  done  after  the  larva  is  inside.  In  two  instances  to  which 
particular  attention  was  given,  the  time  spent  weaving  prior  to 
entering  the  web  was  eight  minutes  in  each  case,  and  the  weaving 
time  inside  the  web  was  56  and  52  minutes  respectively.  In  two 
other  instances  the  larvae  were  weaving  inside  the  web  60  and  70 
minutes  respectively.  The  principal  part  of  the  web  is  woven 
from  the  inside  and  is  supported  on  the  lighter  structure  previ- 
ously woven  from  the  outside.  The  entire  process  of  spinning 
the  web  takes  about  one  or  one  and  one-half  hours,  varying"  some- 
what with  the  larva.  The  procedure  is  essentially  as  described 
by  Snodgrass  for  Bucculatrix  pomifoUclla.  When  the  larvae  are 
numerous,  the  birch  leaves  in  August  and  September  are  spotted 
on  both  sides  with  many  white  webs.  I  call  these  "molting  webs" 
rather  than  "cocoons,"  "pseudococoons,"  or  "cocoonets,"  as 
termed  by  others,  because  I  believe  the  word  "cocoon"  should  be 
restricted  to  that  structure,  in  which  the  pupal  stage  is  passed. 

Having"  completed  its  web,  the  larva  retracts  its  appendages 
somewhat  and  remains  quiescent  a  day  or  two.  The  tarsal  claws 
and  the  crochets  of  the  prolegs  are  not  attached  to  the  web,  the 
larva  lying  freely  with  its  ventral  side  away  from  the  leaf.  If 
the  upper  part  of  the  web  is  removed,  the  larva  falls  out.  Under 
such  conditions  it  must  molt  in  some  sort  of  a  chamber  or  fall  off 
the  leaf,  which  might  be  disastrous,  for  if  food  is  not  available 
after  the  molt,  the  larva  dies  in  a  few  hours.  The  small  size  of 
the  web  holds  the  insect  tightly,  and  the  strong  attachment  to  the 
leaf  secures  the  web  against  being  washed  off  or  lightly  brushed 
off.  The  web  also  offers  protection  from  such  enemies  as  ants 
during  a  period  of  helplessness.  It  is  not  essential  to  the  process 
of  molting,  and  seems  to  be  an  obstacle  to  quick  molting  rather 
than  an  aid.  If  removed  from  the  web,  the  larva  molts  perfectly 
normally.  Inside  the  web  it  has  to  pull  itself  around  to  get  clear 
of  its  old  skin.  After  one  or  two  days  in  the  web,  the  larva  molts, 
and  in  a  few  hours,  sometimes  in  one  hour,  it  breaks  out  through 
the  edge,  at  the  junction  of  "roof"  and  "floor"  (plate  XVII).  In 
molting,  the  head  capsule  separates  from  the  rest  of  the  old  skin 
and  is  cast  off  first  anteriorly.  The  larva  then  works  its  way 
clear  of  the  remaining  skin,  casting  it  off  the  posterior  segment 
of  the  abdomen.  The  molted  head  capsule  and  skin  are  left  inside 
the  web  and  separate  from  each  other. 

The  manner  of  leaving  the  web  shows  how  precisely  instincts 
can  regulate  action.  After  it  has  molted,  the  larva  normally  bites 
a  hole  through  the  side  of  the  web  and  emerges,  but  before  molt- 
ing it  will  not  bite  through  and  hence  cannot  get  out  even  though 
it  so  desires.  An  individual  which  had  just  entered  the  web  (in 
this  case  the  second  molting  web)  was  rendered  inert  by  hydro- 
cyanic acid  gas.  After  four  minutes  it  regained  sensibility  and 
for  the  next  14  minutes  made  spasmodic  movements  while  recov- 


434  CONNECTICUT    EXPERIMENT    STATION  BULLETIN    288 

eringj  being  apparently  normal  at  the  end  of  this  time.  It  then 
attempted  to  get  out  of  the  web  by  pushing  against  the  sides, 
having  "forgotten"  the  reason  for  its  imprisonment.  It  pushed 
vigorously  back  and  forth  for  seven  minutes,  stretching  the  sides 
of  the  web  in  its  endeavors  to  escape,  but  to  no  avail.  Half  a 
minute's  work  with  its  mandibles  would  have  set  it  free,  and  had 
it  molted,  escape  would  have  normally  been  accomplished  in  this 
manner.  The  instinct  to  bite  its  way  out  was  totally  lacking. 
Finally  it  began  to  move  around  inside  the  web  and  spin  irregu- 
larly, then  it  began  to  weave  the  normal  figure  "8"  loops,  and  in 
28  minutes  the  web  was  finished.  The  larva  cannot  use  its  only 
means  of  escape  from  the  web  until  the  act  of  molting  is  accom- 
plished. This  individual  later  molted  and  developed  normally, 
not  being  in  any  way  injured  by  its  treatment. 

If  removed  from  the  web  before  it  is  finished,  or,  if  it  is 
finished,  before  the  pre-molting  quiescent  stage  begins,  the  larva 
will  spin  another  web  or  as  much  of  another  web  as  is  possible 
and  will  molt  normally.  An  effort  is  always  made  to  complete 
another  web,  but  sometimes  lack  of  the  necessary  silk,  or  exhaus- 
tion, or  some  other  factor,  compels  the  larva  to  stop  after  a  few 
strands  have  been  spun,  and  it  then  molts  in  the  most  convenient 
place.  If  it  has  entered  on  the  quiescent  stage  prior  to  molting 
and  has  become  fixed  in  the  shape  of  a  horseshoe,  it  does  not 
straighten  out  when  taken  from  the  web,  but  retains  its  curved 
shape  until  it  molts. 

Because  of  the  fact  that  the  larva  leaves  its  web  so  soon  after 
molting,  the  duration  of  the  instars  has  been  calculated  to  include 
the  time  spent  in  the  web  made  by  the  particular  instar  in  question. 
Thus  the  feeding  period  plus  the  subsequent  quiescent  period  spent 
in  the  web  gives  the  length  of  the  instar. 

The  time  spent  in  the  first  molting  web  is  much  affected  by 
temperature,  and  usually  varies  between  one  and  four  days  in  this 
climate.  Many  larvae  spend  less  than  24  hours  in  this  web,  but 
most  of  the  larvae  are  in  it  about  two  days.  If  this  period  is 
added  to  the  days  of  mining  life,  we  get  a  period  of  24  to  29  days 
for  the  first  three  larval  instars.  This  is  not  remarkably  long 
when  compared  to  the  length  of  the  next  two  instars,  which 
together  total  about  two  weeks. 

After  emerging  from  its  first  molting  web,  the  larva  feeds  from 
one  to  nine  days,  the  individuals  varying  greatly  under  the  same 
conditions.  If  food  is  withheld  from  the  newly  molted  larva,  it 
dies  in  a  few  hours,  a  much  shorter  time  than  if  starved  after 
feeding  a  day  or  two.  This  is  probably  the  result  of  remaining 
a  day  or  two  in  the  molting  web  without  feeding.  During  the 
fourth  instar  the  larvae  are  restless  and  wander  about  more  or 
less.  This  probably  accounts  in  part  for  the  variation  in  the 
length   of   the   instar,    for   the   rapidity   of   development   is   much 


BIOLOGY    OF    BIRCH    LEAF    SKELETONIZER  435 

dependent  on  the  amount  of  food  eaten.  The  average  duration 
of  the  feeding"  period  for  73  individuals  recorded  was  about  four 
days.  In  only  one  instance  was  the  feeding  period  as  short  as  one 
day.  Temperature  affects  the  duration  of  this  period  to  some 
extent,  as  will  be  brought  out  later.  The  effect  of  different  spe- 
cies of  birch  as  food  will  also  be  discussed  in  another  section  of 
this  paper. 

The  feeding  occurs  normally  on  the  lower  side  of  the  leaf,  and 
the  veins  and  the  upper  epidermis  are  left  intact.  The  entire  leaf 
is  never  consumed.  It  is  due  to  this  habit  of  skeletonizing  a  leaf 
that  the  insect  bears  its  common  name.  The  larvae  will  eat  which- 
ever surface  of  the  leaf  is  toward  the  ground,  and  normally  this 
is  the  lower  epidermis.  A  birch  leaf  was  inverted  so  that  the 
normal  lower  surface  was  uppermost  and  covered  with  a  black 
paper.  A  mirror  was  so  placed  that  it  reflected  light  on  the  leaf 
from  below.  The  larvae  normally  feed  on  the  lower  side  of  the 
leaf,  and  under  normal  conditions  this  side  is  not  so  light  as  the 
upper.  If  the  larvae  fed  on  the  lower  side  of  this  inverted  leaf, 
they  would  feed  on  the  lighter  side  and  at  the  same  time  on  the 
side  normally  uppermost.  The  two  sides  of  the  leaf  differ  in 
physical  as  well  as  chemical  constitution  of  the  surface.  Of  ten 
larvae  placed  on  the  upper  side  of  this  inverted  leaf,  four  migrated 
to  other  parts  of  the  plant  (a  normal  movement),  one  remained 
on  the  upper  side  and  was  feeding  when  examined,  and  five  went 
to  the  lower  side  of  the  leaf  and  were  feeding.  Seventeen  hours 
elapsed  between  the  placing  of  the  larvae  on  the  leaf  and  the  final 
observation.  Larvae  were  then  placed  on  the  uppermost  side  of 
an  inverted  leaf  and  watched.  Usually  they  wandered  about 
restlessly  for  a  time  until  they  came  to  the  edge  of  the  leaf.  They 
then  turned  to  the  side  underneath.  Light  reflected  on  the  lower 
surface  by  a  mirror  seemed  to  have  no  effect.  At  times  move- 
ment to  the  lower  surface  was  long  delayed  and  at  times  direct. 
It  very  evidently  is  a  reaction  to  gravity  that  impels  these  larvae 
to  feed  on  the  lower  leaf  surface  and  not  any  dislike  for  the  upper 
surface  nor  any  negative  reaction  to  bright  light.  What  factors 
developed  the  habit  of  feeding  on  the  lower  surface  only  is  another 
matter.  The  habit  of  the  larva  is  to  feed  continuously  over  a  lim- 
ited area,  and  it  does  not  wander  far  unless  the  food  supply  gives 
out.  If  disturbed,  the  larva  usually  drops  off  the  leaf,  spinning 
a  long  thread  as  it  falls.  After  falling  a  few  inches  it  hangs  on 
the  end  of  the  thread  a  moment  and  then  quickly  ascends.  The 
thread  is  spun  out  the  tip  of  the  spinneret,  and  when  the  larva 
stops  its  descent,  it  is  attached  to  the  end  of  the  thread  by  means 
of  the  spinneret.  When  it  ascends  the  thread,  it  moves  its  head 
rapidly  back  and  forth  and  winds  the  silk  on  the  prothoracic  legs 
which  are  held  forward.  If  there  is  too  much  silk  for  the  pro- 
thoracic  legs,  the  mesothoracic  legs  are  brought  into  use.     On 


43^  CONNECTICUT    EXPERIMENT    STATION  BULLETIN    288 

regaining  its  support,  the  larva  simply  drops  the  bundle  of  thread 
and  walks  away.  This  performance  can  be  easily  watched  under 
the  binocular  if  a  larva  of  the  last  instar  is  used.  The  spinning 
activities  of  the  larva,  the  quickness  with  which  it  drops  from  a 
leaf,  and  the  distance  it  drops  are  much  greater  in  the  last  instar 
than  in  the  fourth.  The  speed  with  which  these  little  insects  can 
spin  a  thread  while  falling  a  few  feet  is  remarkable.  If  touched, 
they  snap  the  body  back  and  forth  rapidly  and  thus  wriggle  off 
the  leaf  and  drop  toward  the  ground.  Yet  after  they  have  fallen 
some  distance,  they  suddenly  check  their  descent  and  can  be  seen 
to  be  hanging  by  the  end  of  a  thread.  The  silk  of  which  this 
thread  is  formed  must  be  spun  from  the  silk  glands  and  out  of 
the  spinneret  as  rapidly  as  the  larva  falls.  The  act  of  spinning 
apparently  occurs  automatically  when  the  larva  is  disturbed. 

Because  of  their  small  size  and  their  greenish  color,  together 
with  the  comparatively  small  amount  of  leaf  tissue  eaten,  larvae 
of  the  fourth  instar  are  not  so  noticeable  as  those  following.  In 
localities  where  Bucculatrix  is  abundant,  however,  ten  to  fifteen 
larvae  may  often  be  found  on  one  leaf.  Heavily  infested  birches 
frequently  have  25  larvae  of  the  fourth  and  fifth  instars  feeding 
on  each  leaf.  During  the  majority  of  seasons  no  such  number  is 
likely  to  be  present. 

The  fourth  instar  molts  as  did  the  third,  in  a  white  silken  web. 
This  web  is  larger  than  the  previous  one,  being  about  2.5  mm. 
across.  The  larva  builds  the  web  and  lies  in  it  as  previously 
described,  being  clearly  visible.  There  is  a  slight  difference  in 
structure,  as  this  larva  weaves  an  elliptical  mat  after  it  is  inside 
the  web.  This  thickened  part  gives  the  second  molting  web  a 
characteristic  appearance,  as  the  first  molting  web  has  this  struc- 
ture to  only  a  very  slight  degree.  The  time  spent  in  this  web 
varies  normally  from  one  to  three  days,  the  75  individuals 
recorded  averaging  about  two  days.  This  is,  of  course,  affected 
by  the  temperature,  as  was  mentioned  before.  When  added  to 
the  feeding  period  this  figure  gives  the  length  of  the  fourth  instar 
as  about  six  days. 

The  larva  molts  as  before  and  emerges  from  the  second  molting 
web  as  from  the  first.  It  normally  feeds  on  the  under  side  of  the 
leaf,  skeletonizing  it  (plate  XVIII),  and  in  this  instar  the  feeding 
is  much  more  extensive.  The  injury  to  the  foliage  is  most  notice- 
able at  this  time,  usually  during  the  last  of  August  and  most  of 
September.  If  the  larvae  are  present  in  large  numbers,  all  the 
parenchymatous  tissue  is  consumed,  and  the  leaf  dies  and  drops 
from  the  tree-  These  larvae  show  greater  spinning  activity  than 
those  of  the  former  instar  and  may  be  seen  suspended  from  the 
leaves  in  great  numbers  in  seasons  of  abundance.  They  feed 
from  two-  to  ten  days,  the  period  varying  with  the  individual  and 
being  affected  by  climatic  conditions,  and  an  average  of  48 
recorded  individuals  gives  a  period  of  nearly  seven  days.     This 


BIOLOGY    OF    BIRCH    LEAF    SKELETONIZER  437 

period  includes  the  time  from  emergence  from  the  second  molting" 
web  to  the  spinning  of  the  cocoon.  Toward  the  last  part  of  the 
feeding  period  the  gonads  are  clearly  seen  through  the  dorsal  skin 
of  the  abdomen  of  the  larva.  About  twelve  hours  before  the  time 
when  the  larva  will  begin  to  spin  its  cocoon,  it  stops  feeding. 
This  interval  of  time  varies  considerably  and  may  be  much  less. 
By  this  time  the  larva  has  turned  brown  in  color,  due  to  the  color 
of  the  large  silk  glands  which  run  almost  the  entire  length  of  the 
body.  When  ready  to  spin  the  cocoon,  the  larva  drops  from  the 
place  of  feeding  to  the  ground,  spinning  out  a  long  thread  as  it 
goes.  Larvae  may  sometimes  be  seen  suspended  from  a  thread 
about  fifteen  feet  long.  If  the  trees  on  which  they  are  feeding- 
are  shaken,  these  fully  grown  larvae  drop  to  the  ground  quickly 
and  in  considerable  numbers.  Having  reached  the  ground,  they 
crawl  under  a  stone,  a  fallen  branch,  a  leaf,  or  any  other  object 
lying  on  the  ground  and  spin  their  cocoons  on  the  under  side  of 
this.  Sometimes  the  cocoon  is  spun  on  the  ground  itself.  In 
captivity  they  will  frequently  place  the  cocoons  on  the  sides  of  the 
cage  close  to  the  base.  I  have  rearea  hundreds  of  larvae,  and 
they  all  have  dropped  to  the  ground  or  close  to  it  to  pupate. 
Fletcher  (1893)  mentions  finding  three  cocoons  on  the  twig  of  a 
birch,  but  all  the  cocoons  which  I  have  found  in  the  field  have 
been  on  fallen  leaves  or  other  objects  lying  on  the  ground. 

The  manner  in  which  the  larva  spins  its  cocoon  is  characteristic 
of  the  genus  and  quite  unique.  The  earliest  description  of  this 
process  in  the  genus  Bucculatrix  is  by  Lyonet,  who  wrote  to 
Reaumur,  December  22,  1744,  concerning  the  larva  of  B.  ulmella 
and  its  cocoon.  This  description  was  not  published  until  1832  and 
has  been  referred  to  in  the  historical  part  of  this  paper  (page  396). 
De  Geer,  in  the  first  volume  of  his  "Memoires,"  published  in  1752, 
described  the  cocoon  of  B.  frangulella  (see  page  395),  and  Snod- 
grass  in  1922  likewise  described  the  manner  in  which  B.  pomi- 
foliella  Clemens  wove  its  cocoon.  These  three  papers  go  into  the 
details  of  the  process  by  which  the  larva  lays  down  its  threads, 
and  from  a  microscopic  examination  of  the  cocoon  of  B.  canaden- 
sisella it  is  apparent  that  this  larva  weaves  its  threads  in  precisely 
the  same  manner  as  does  B.  pomifoliella.  The  general  process 
of  weaving  is  similar  in  all  four  species,  differing  only  in  a  few 
details.  Chambers  (1882)  described  briefly  the  formation  of  the 
cocoon  by  B.  ambrosiaefoliella  Chambers,  and  McGregor  (1916) 
gave  a  brief  description  of  the  finished  cocoon  of  B.  thurberiella 
Busck.  In  1892  Fletcher  briefly  described  the  general  procedure 
of  weaving  by  B.  canadensisella  Chambers,  and  in  1893  Lintner 
mentioned  the  same  subject,  but  the  latter's  description  is  not 
correct,  and  Fletcher's  description  is  not  detailed. 

The  larva  of  B.  canadensisella  Chambers  first  lays  down  an  oval 
mat  to  serve  as  a  base  for  its  cocoon.  It  does  not  previously 
weave  a  palisade  of  poles  around  the  site  selected,  as  do  many 


43&  CONNECTICUT    EXPERIMENT    STATION  BULLETIN    288 

species  of  the  genus.  It  then  commences  at  one  end  of  the  mat 
to  weave  an  outer  supporting"  ridged  structure  of  comparatively 
coarse  threads  (about  .005  mm.  thick),  facing  the  work  and  back- 
ing away  as  the  woven  structure  progresses  over  the  mat  in  an 
arch.  The  ridges  are  formed  by  the  ends  of  a  series  of  loops 
made  from  one  side  to  the  other.     The  diagram  in  figure  32  gives 


Fig.  32.     Diagram  of  method  by  which  larva  of  Bucculatrix  pomifoliella 
Clemens  weaves  its  cocoon.     After  Snodgrass. 


the  principle.  Between  ridges  the  threads  cross  diagonally.  This 
figure  is  from  Snodgrass  1(1922)  and  gives  his  conception  of  the 
actual  motions  made  in  weaving.  As  the  cocoon  becomes  higher, 
the  larva  raises  the  anterior  part  of  its  body,  and  the  radius  of 
the  structural  arch  is  gauged  by  the  raised  part  of  the  body  as 
it  swings  from  side  to  side,  most  of  the  body  being  fixed  in  the 
midline  of  the  oval  base.  Possibly  the  prothoracic  legs  are  used 
in  the  weaving  to  aid  in  guiding  the  work,  as  mentioned  by  Snod- 
grass and  Lyonet.  Certainly  these  legs  are  held  up  to  the  struc- 
ture. When  the  cocoon  is  about  two-thirds  finished,  the  larva 
enters  it,  turns  about,  and  crawls  out  until  its  head  reaches  the 
other  end  of  the  mat.  It  now  has  its  anterior  end  outside  of  the 
cocoon  but  its  posterior  end  in' the  cocoon.  Beginning  to  weave 
exactly  as  before,  the  larva  builds  up  the  last  third  of  the  cocoon 
to  meet  the  previously  formed  two-thirds,  gradually  enclosing 
itself  as  it  works.  When  the  two  sections  meet,  they  are  joined 
by  cross  threads.  The  architecture  is  not  perfect,  for  the  ridges 
of  the  two  sections  rarely  coincide,  and  sometimes  the  heights  of 
the  sections  are  not  equal.  The  result  is  a  break  in  the  continuity 
of  the  ridges  at  the  junction  and  often  a  sag  in  the  contour  of  the 
cocoon.  A  completed  cocoon  is  shown  on  plate  XVIII.  This 
outer  structure  is  not  closely  woven  and  the  insect  can  be  seen 
clearly  inside.  It  is,  however,  stiff  and  gives  support  to  the  lining 
which  is  to  be  woven.  The  sides  meet  the  oval  base  perpen- 
dicularly. 

Having  completed  its  superstructure,  the  larva  weaves  a  closely 
knit  lining  of  fine  threads  (about  half  the  thickness  of  the  threads 
of  the  supporting  structure)  all  around  the  inside  by  swinging 
its  head  in  figure  "8"  loops.  Where  the  walls  of  the  superstruc- 
ture join  the  base,  the  cocoon  does  not  follow  but  makes  a  round 


BIOLOGY    OF    BIRCH    LEAF    SKELETONIZER  439 

corner,  as  the  diagram  in  figure  31  shows.  It  is  this  lining"  which 
makes  the  cocoon  opaque.  Snodgrass  has  described  a  series  of 
"valves"  in  the  anterior  end  of  the  cocoon  of  B.  pomifoliella 
Clemens,  but  in  the  cocoon  of  B.  canadensis ella  these  are  not  pres- 
ent. The  cocoon  when  first  finished  is  almost  pure  white,  but  it 
soons  turns  brown.  This  brown  color  is  due  not  to  the  pupa 
inside,  for  it  is  present  before  the  prepupa  molts,  but  to  a  change 
in  the  color  of  the  silk  when  exposed  to  air.  The  time  necessary 
to  complete  a  cocoon  is  from  eight  to  sixteen  hours  normally. 
Inside  the  cocoon  the  larva  remains  two  or  three  days  before 
pupating.  This  prepupal  period  plus  the  feeding  period  makes 
the  fifth  instar  about  nine  days  long  on  the  average.  The  larva 
molts  in  the  cocoon  in  a  manner  differing  slightly  from  that  which 
takes  place  in  the  molting  webs.  In  the  webs  the  head  capsule  is 
cast  off  entire  and  anteriorly  while  the  rest  of  the  larval  skin  is 
worked  posteriorly  off  the  anal  segment.  In  the  cocoon  the  entire 
larval  skin,  head  capsule  included,  is  worked  off  posteriorly. 

The  individuals  which  were  reared  in  the  outdoor  insectary 
under  normal  temperatures  in  1924  pupated  from  September  4th 
to  September  25th;  in  1925  from  September  8th  to  September 
13th;  in  1926  from  September  nth  to  September  23d.  This  does 
not  indicate  the  time  of  disappearance  of  the  last  larvae  in  the 
field.  During  these  three  years  an  examination  of  birches  about 
New  Haven  was  made  in  order  to  determine  the  normal  close  of 
the  larval  period.  In  1924  the  last  larvae  were  found  October 
9th;  in  1925,  September  19th;  and  in  1926,  October  9th.  The 
early  disappearance  of  larvae  in  1925,  although  not  caused  by  any 
apparent  natural  enemy  or  unusual  climatic  condition,  was  excep- 
tional. It  may  have  been  caused  in  part  by  an  early  season  start- 
ing the  life  cycle  earlier.  In  view  of  the  fact  that  larvae  will  feed 
at  480  to  50°F.  and  will  eat  birch  leaves  until  they  begin  to  turn 
yellow  very  few  are  caught  before  pupation  by  cold  weather  or 
lack  of  food. 

The  total  larval  life  occupies  from  38  to  46  days,  as  a  rule,  as 
the  table  on  oaee  441  indicates.  This  is  not  an  average  of  the  com- 
pleted larval  life  of  a  number  of  insects,  but  an  average  of  the 
separate  stages  of  many  individuals,  rather  few  of  which  com- 
pleted the  entire  larval  period  while  under  observation.  Nine 
larvae  carried  through  from  egg  to  pupa  in  1926  averaged  41 
days,  the  maximum  being  45  days,  and  the  minimum  36  days. 
This  is  as  close  as  could  be  expected  to  the  38  days  given  in  the 
summary  for  1926.  In  the  table  below,  the  larval  life  from  the 
hatching  of  the  egg  to  the  spinning  of  the  cocoon  is  given  for  the 
nine  individuals  mentioned  above.  Two  days  as  prepupa  should  be 
added  to  the  six  days  of  feeding  in  the  fourth  instar  to  give  the 
total  larval  period  of  41  days.  It  will  be  noticed  that  the  larva' 
does  not  accelerate  through  one  instar  if  slowed  down  on  a  previous 


44° 


CONNECTICUT    EXPERIMENT    STATION  BULLETIN    288 


instar,  but  that  any  retardation  during  the  growing  period  is  per- 
manent as  regards  time.  This  is  borne  out  by  the  other  records. 
Seven  larvae  were  reared  in  the  laboratory  in  vials  containing 
moist  sand,  and  were  under  identical  environmental  conditions. 
The  figures  for  the  stages  are  given  on  pages  474-478  (larvae  131- 
137).  The  most  slowly  growing  larva  was  six  days  in  the  fourth 
instar  and  was  feeding  six  days  and  fifteen  hours  in  the  fifth, 
while  the  most  quickly  growing  larva  was  four  days  and  nine 
hours  in  the  fourth  instar  and  was  feeding  four  days  and  nineteen 
hours  in  the  fifth.  It  will  also  be  noticed  that  the  quiescent  period 
spent  in  the  molting  web  is  independent  of  the  length  of  the  feed- 
ing period,  and  as  the  feeding  period  grows  shorter,  the  propor- 
tion of  time  spent  in  the  web  during  one  instar  grows  greater.  In 
larva  number  9  in  the  table  below,  two-fifths  of  the  fourth  instar 
is  quiescent,  and  in  number  1,  three-sevenths,  but  in  numbers  2 
and  4.  only  one-fourth  of  the  fourth  instar  is  quiescent.  The 
effect  of  food  and  temperature  on  larval  growth  will  be  discussed 
later. 


Table 

i.     Complete 

Larval  Pi 

:riod 

to 

£    1 

™ 

■0 

% 

-a 

bo 

j= 

ho 

c 

P 

.a 

2 

£ 

"rt 

T3 

£ 

"a 

•3 

s-.H 

"(3 

a 

»— 1 

0 

1) 

s 

0 

0 

£ 

ffl 

h- 1 

H 

h 

H 

to 

tn  O 

H 

I 

8-14-26 

26 

2 

28 

4 

3 

7 

S 

9-23-26 

40 

2 

8- 1 1-26 

22 

2 

24 

6 

2 

8 

7 

9-19-26 

39 

3 

8- 1 1 -26 

22 

3 

25 

4 

2 

6 

6 

9-17-26 

37 

4 

8-14-26 

20 

2 

22 

6 

2 

8 

6 

9-19-26 

36 

a 

8-  7-26 

27 

2 

20 

4 

2 

6 

5 

9-16-26 

40 

6 

8- 1 1 -26 

30 

1 

31 

5 

3 

8 

4 

9-23-26 

43 

7 

8-  7-26 

27 

3 

30 

5 

2 

7 

6 

9-19-26 

43 

8 

8-14-26 

20 

2 

22 

S 

2 

7 

6 

9-18-26 

35 

9 

8-  7-26 

22 

1 

23 

3 

2 

5 

6 

9-10-26 

34 

Aver. 

26±.74 

7±.22 

6±.i6 

39^-7 

The  chart  below  (text  figure  33)  gives  the  periods  during  which 
the  various  stages  may  be  found  in  the  field  around  New  Haven, 
Connecticut.  These  limits  are  computed  from  field  observations 
and  data  obtained  in  the  insectary  and  are  broader  than  actual 
field  observation  alone  would  give.  From  what  notes  there  are 
of  the  occurrence  of  this  insect  elsewhere,  it  seems  likely  that  these 
periods  are  approximately  correct  for  the  entire  region  in  which 
the  insect  is  found. 


BIOLOGY    OF    BIRCH    LEAF    SKELETONIZER 


441 


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442 


CONNECTICUT    EXPERIMENT    STATION  BULLETIN    288 


VII.     Determination  of  the  Number  of  Instars 

It  is  very  difficult  to  determine  the  number  of  instars  by  exam- 
ining the  mines  for  head  capsules.  A  large  number  of  mines 
were  examined  for  this  purpose  and  in  two  cases  one  capsule  was 
found.  The  extent  of  growth  and  the  morphological  changes 
undergone  during  the  mining  period  indicated  at  least  two  and  per- 


Fig.  33.  _  Seasonal  occurrence  of  the  various  stages  of  B.  canadensisella 
in  the  vicinity  of  New  Haven.  The  larval  periods  shown  should  read 
mining  instars,  fourth  instar  and  fifth  instar. 


haps  three  larval  stages.  A  number  of  larvae  were  collected  in  the 
field  and  the  width  of  the  heads  measured.  The  head  is  not  sub- 
ject to  growth  changes  during  any  one  instar,  and  according  to 
Dyar  (1890)  a  constant  numerical  ratio  exists  between  the  widths 
of  the  heads  of  any  two  successive  instars  of  a  larva.  If  the  heads 
of  two  successive  instars  are  measured,  or  if  a  large  number  of 
miscellaneous  heads  are  measured,  the  ratio  for  the  species  can  be 
determined  and  the  possibility  of  missing  an  instar  removed.  Any 
dimension  of  the  head  may  be  used,  but  the  width  is  the  most 
convenient. 

Several  embryos  which  had  developed  to  the  stage  where  they 
were  about  to  emerge  from  the  egg  and  where  no  further  growth 
of  the  head  could  be  expected  were  measured.  These  were  all 
mounted  in  Canada  balsam.  As  seen  by  the  table  on  page  444, 
the  average  width  is  .078  mm.,  and  nine  of  the  twelve  measured 
.076  mm.,  which  latter  figure  may  be  considered  normal.  It  is  to 
be  expected  that  the  measurements  for  the  first  instar  would  con- 
form to  this  figure,  and  of  the  sixty-one  mining  larvae  measured, 
eighteen  either  equal  this  figure  or  closely  approximate  it.  All 
but  two  of  the  eighteen  equal  it.  The  average  width  for  the  first 
instar  is  then  .077  mm.     Thirty-six  of  the  sixty-one  measure  .114 


BIOLOGY    OF    BIRCH    LEAF    SKELETONIZER  443 

mm.  in  width  or  very  nearly  so,  thirty-four  measuring"  just  that 
figure  and  the  other  two  measuring  .120  mm.  The  normal  and 
average  for  this  group  is  .114  mm.  The  remainder  of  the  mining 
larvae  measured,  thirteen,  all  give  a  head  width  of  .171  mm. 
Two  larvae  were  secured  just  as  they  left  the  mine  and  before  they 
began  to  weave  the  molting  web,  and  their  heads  measured. 
Both  gave  a  width  of  .171  mm.  These  two  are  marked  (ex)  in 
the  third  column.  This  checked  the  group  giving  this  measure- 
ment as  the  last  mining  instar.  Also  four  larvae  were  found  in 
the  process  of  molting  and  with  the  head  capsules  just  far  enough 
off  to  permit  the  measurement  of  both  the  old  capsule  and  the  new 
head.  Two  of  these  gave  the  width  of  the  old  capsule  as  .076 
mm.  and  the  new  head  as  .114  mm.,  while  the  other  two  gave 
.114  mm.  and  .171  mm.  for  the  two  widths.  This  gives  a  check 
on  the  three  groups.  According  to  Dyar's  principle  we  should 
expect 

.ii4_.i7T__R 

.077_.ii4_ 

In  this  case  the  ratio  "R  "  is  1.5,  and  the  number  of  instars  in 
the  mine  is,  as  the  figures  indicate,  three.  To  further  check  this 
principle,  a  number  of  external  feeding  larvae,  also  collected  in 
the  field,  were  measured.  I  have  placed  these  forty-one  larvae  in 
two  groups  as  the  table  shows.  According  to  the  principle  used 
above,  the  measurements  should  be  ,257  mm.  (.171  x  1.5  =  .257) 
and  .385  mm.  (.257x1.5)  for  the  fourth  and  fifth  instars. 
(The  actual  number  of  externally  feeding  instars  was  determined 
by  actual  observation,  of  course.)  In  the  fourth  instar  the  aver- 
age width  was  found  to  be  .245  mm.  for  the  nineteen  individuals, 
with  a  variation  between  .228  mm.  and  .257  mm.  The  last  instar, 
containing  twenty-two  individuals,  gave  an  average  width  of 
•353  nim.  with  a  variation  extending  from  .304 'mm.  to  .390  mm. 
It  is  questionable  whether  the  two  larvae  whose  head  widths  are 
.304  mm.  belong  to  the  fourth  or  the  fifth  instars  if  one  judges 
by  these  two  measurements  alone.  The  average  width  is  less  than 
that  expected  in  both  the  external  feeding  instars,  but  even  so  the 
measurements  are  sufficiently  closely  grouped  in  each  case  to 
determine  the  instar.  It  is  to  be  expected  that  the  more  nearly 
the  larvae  approach  the  fully  grown  condition,  the  more  widely 
will  they  vary  in  size,  for  the  absolute  extent  of  variation  in  size 
under  normal  conditions  increases  with  age.  The  change  in 
environment  from  the  mine  to  the  surface  of  the  leaf,  with  its 
difference  in  manner  of  feeding  involved,  would  also  change  the 
shape  of  the  head,  because  mining  larvae  have  relatively  flatter 
heads..  The  actual  measurements  obtained  of  the  heads  of  the 
first  three  instars  is  much  closer  to  the  ideal  than  would  usually 
be  expected. 


444 


CONNECTICUT    EXPERIMENT    STATION  BULLETIN    288 


Table  3.     Head  Widths  of  the  Larvae  of  B.  canadensisella  Chambers 
(All  dimensions  in  millimeters) 


First                  Second 

Third 

Fourth                Fifth 

Embryo 

instar 

instar 

instar 

instar                instar 

I 

.076° 

.076* 

114° 

.i7i°(ex) 

.257 

.323' 

2 

.076° 

.076* 

114° 

.171°  (ex) 

.247 

.352' 

3 

.076° 

.076* 

1 140 

.171° 

.247 

.323' 

4 

.076° 

.076* 

120° 

.171° 

.228 

•304' 

s 

.086° 

.076* 

1 140 

.171° 

.247 

.371' 

6 

.076° 

.076* 

1140 

.171* 

.238 

.380' 

7 

.086° 

.076* 

1 140 

.171* 

.247 

.38o' 

8 

.082° 

.076* 

1140 

.171* 

.247 

•300' 

9 

.076° 

.076* 

1 140 

.171* 

.228 

.380' 

10 

.076° 

.076* 

114* 

.171' 

.247 

.371' 

11 

.076° 

.076* 

114* 

.171 

.238 

.36l' 

12 

.076° 

.076'* 

114* 

.171' 

.247 

.361' 

13 

.076* 

114* 

.171 

.247 

.304' 

14 

.095* 

114* 

.171' 

.247 

.370' 

15 

.076' 

114* 

.171' 

.252 

.38o' 

16 

.082] 

114* 

.228 

.352' 

17 

.076' 

114* 

.250 

.361' 

18 

.076' 

120* 

•257 

.361' 

19 

114* 

.247 

■352' 

20 

114* 

.332' 

21 

114* 

.380' 

22 

114* 

•370' 

23 

114* 

24 

114* 

25 

114* 

26 

1140 

27 

1 140 

28 

114° 

29 

114 

30 

114' 

3i 

114' 

32 

114' 

33 

114' 

34 

114' 

35 

114' 

36 

114' 

Theoretical  average 

114 

.171 

.257 

.385 

Average  found 

.078  ± 

.0008  .o77±.oooi 

U4±oooi 

.i7i±o.o 

•245 

±.OOI3    .353=! 

Standard  deviation 

.0039 

.0045 

001 

.000 

.0085 

.025 

Greatest  deviation 

from  theoretical 

.008 

.008 

006 

.000 

.017 

.081 

.0036 


111  previous  descriptions  of  the  genus  Bucculatrix  it  has  been 
tacitly  assumed  or  explicitly  stated  that  the  mining"  period 
included  one  instar  only,  and  that  the  insect  always  molted  on  the 
surface  of  the  leaf.  The  only  mention  I  have  found  of  a  larva 
molting"  in  the  mine  is  in  a  description  of  the  larva  of  B.  ambro- 
siaefoliclla  Chambers  by  Chambers  (1882)  in  which  he  states  that 
the  larva  in  question  molts  once  in  the  mine,  once  on  the  surface 
of  the  leaf,  and  once  in  the  cocoon.  It  would  be  well,  however, 
to  apply  Dyar's  principle,  at  least  to  the  early  stages,  before  mak- 
ing any  definite  statements  regarding  other  species  of  this  genus. 


BIOLOGY    OF    BIRCH    LEAF    SKELETONIZER  445 

All  the  measurements  given  above  were  made  with  an  ocular 
micrometer,  using'  a  low  power  of  the  microscope.  The  smallest 
micrometer  scale  division  was  .019  mm.,  and  it  was  found  imprac- 
ticable to  interpolate  to  less  than  one-fourth  this,  a  measurement 
of  .005  mm.  The  embryos  measured  were  all  mounted  in  Canada 
balsam.  The  larvae  of  the  first  three  instars,  marked  with  a  small 
circle,  :'°,"  were  also  mounted  in  balsam ;  those  marked  with 
an  asterisk,  "*,"  were  mounted  in  glycerine;  and  those  marked 
with  an  apostrophe,  '","  were  specimens  preserved  in  alcohol. 
All  the  fourth  and  fifth  instar  larvae  were  preserved  in  alcohol 
after  fixation  in  Gilson-Carnoy's  fluid. 

VIII.     Food  Plants 

The  plants  on  which  the  larvae  feed  are  restricted  to  the  genus 
Bctula,  with  the  possible  exception  of  the  alder,  Alnus  incana. 
Johannsen,  who  reports  (1911)  the  single  instance  of  larvae 
attacking  the  alder,  has  also  reported  (1910)  the  presence  of 
larvae  on  red  oak.  There  are  other  species  of  Bncculatrix  which 
feed  on  oak,  one  of  which  is  very  common  in  Connecticut,  and  it 
is  very  probable  that  the  larvae  referred  to  by  Johannsen  were 
not  B.  canadensisella.  The  mines  in  oak  leaves  are  very  similar 
to  those  of  the  birch  skeletonizer,  but  the  cocoons  are  white  and 
are  found  on  the  trunk  and  branches  of  the  trees.  I  have  not 
bred  this  species,  but  Forbes  (1923)  gives  B.  ainsliella  Murtfeldt 
and  B.  packardeila  Chambers  as  indigenous  to  northeastern  United 
States,  and  both  feed  on  oak.  Alder  is  closely  related  to  birch, 
and  although  the  larvae  of  B.  canadensisella  did  not  survive  in 
laboratory  tests,  through  one  complete  instar  on  Alnus  (rugosaf), 
under  different  conditions  they  may  possibly  feed  on  this  plant. 
Of  the  species  of  birch  on  which  this  insect  lives,  four  are  native 
and  one  imported  from  Europe.  These  are  Betula  popalifoUa 
(gray  birch),  B.  papyrifera  (paper  or  white  birch),  B.  lutea  (yel- 
low birch),  B.  lenta  (black  birch),  and  B.  alba  (European  white 
birch)  respectively.  The  European  birch  is  a  common  ornamen- 
tal tree  in  northeastern  United  States  and  southeastern  Canada, 
and  varieties  are  called  the  cut-leaf  or  weeping  birch.  This  tree 
in  Canada  seems  to  be  a  favorite  food  plant,  but  in  the  vicinity 
of  New  Haven  it  is  not  quite  so  severely  attacked  as  the  gray 
birch. 

Of  the  four  native  food  plants,  the  black  birch  seems  to  suffer 
least,  although  Maheux  reports  (1926)  that  in  Quebec  this  tree 
has  been  heavily  skeletonized.  Which  of  the  other  three  is  most 
severely  injured  seems  to  depend  on  which  is  prevalent  in  the 
locality.  In  most  of  Connecticut  the  gray  birch  is  the  preferred 
food  plant,  but  on  the  shores  of  Highland  Lake,  where  the  white 
and  black  birches  are  the  only  two  species  common,  the  white 
birches  were  heavily  skeletonized  in  1925  ;    and  in  other  parts  of 


44^  CONNECTICUT    EXPERIMENT    STATION  BULLETIN    288 

Litchfield  County,  where  yellow  birch  is  quite  common,  it  is  a 
favorite  host.  In  Ontario  and  throughout  the  Great  Lakes 
regions,  the  yellow  and  white  birches  are  the  trees  which  suffer 
most.  The  black  birch  in  Connecticut  is  very  slightly  injured  and 
usually  is  untouched,  even  though  its  branches  intermingle  with 
those  of  the  white  and  gray  birches  when  these  two  bear  thousands 
of  caterpillars.  In  laboratory  tests  the  larvae  ate  the  leaves  of 
the  black  birch  very  readily.  These  larvae  were  taken  from  gray 
birch  and  fed  on  black  birch  during  the  fifth  instar.  Five  of  the 
ten  larvae  pupated  normally,  although  the  duration  of  the  instar 
was  173  hours  on  the  average  as  compared  with  117  hours  for  the 
control.  This  delay  in  maturing  was  partly  due  to  the  delay  the 
larvae  experienced  in  getting  accustomed  to  the  new  food  plant. 

The  red  or  river  birch  (Betula  nigra)  is  not  a  common  tree  in 
northern  United  States  and  southern  Canada,  and  this  may  be  the 
reason  that  it  is  not  reported  as  being"  attacked  by  this  insect. 
New  England  is  about  its  northernmost  range,  and  here  it  is 
found  only  in  a  few  scattered  places  along  river  banks.  .  No 
attempt  was  made  to  rear  the  larvae  on  the  leaves  of  this  tree,  as 
the  material  is  not  readily  available,  and  there  are  no  references 
in  the  literature  to  it  as  a  food  plant. 

There  are  four  other  genera  of  plants  belonging  to  the  same 
family  as  the  birches  and  growing  very  commonly  in  the  same 
localities  as  these  trees.  These  are  Ostrya  (hop  hornbeam), 
Carpinus  (ironwood),  Alnus  (alder),  and  Corylus  (hazelnut). 
Under  natural  conditions  I  have  never  observed  any  of  these 
plants  attacked  by  the  larvae  of  B.  canadensisella,  although  they 
very  frequently  intermingle  with  the  birches.  In  the  laboratory 
the  larvae  have  been  forced  to  eat  the  leaves  of  Alnus  but  could 
not  maintain  themselves  on  these  leaves.  The  larva  itself  has 
really  very  little  to  do  with  the  choice  of  food  plants,  for  this  is 
a  leaf -mining  insect  in  the  early  stages,  and  if  the  egg  is  not  laid 
on  a  leaf  in  which  the  larva  can  live,  death  results.  Even  during 
the  external-feeding  stages  it  is  very  questionable  if  a  larva  could 
survive  long  enough  to  travel  from  an  unfavorable  to  a  favorable 
plant  unless  the  two  plants  were  very  close  together. 

In  an  attempt  to  secure  eggs  on  the  leaves  of  the  alder,  I  placed 
two  alder  twigs,  each  bearing  two  or  three  leaves,  in  a  cage  with 
five  males  and  five  females.  One  of  the  twigs  had  been  dipped 
in  the  distillate  from  an  aqueous  extract  of  birch  leaves,  and  the 
other  was  normal.  The  moths  were  collected  in  the  field.  Two 
females  lived  six  days,  one  five  days,  one  three  days,  and  one  two 
days,  but  no  eggs  were  laid.  In  another  similar  trial  with  one 
female  and  six  males,  the  female  lived  four  days  but  laid  no  eggs. 
In  view  of  the  fact  that  the  females  are  loath  to  lay  eggs  in  cap- 
tivity, the  results  are  merely  indicative  and  not  conclusive. 


BIOLOGY    OF    BIRCH    LEAF    SKELETONIZER  447 

An  attempt  was  made  to  force  larvae  to  eat  the  leaves  of  the 
alder  and  the  black  oak.  All  these  larvae  were  collected  in  the 
field  on  gray  birch.  Five  larvae  in  the  first  molting  webs  were 
placed  in  vials  with  the  leaves  of  each  plant.  On  the  alder  all 
five  larvae  died  in  three  and  one-half  days  or  less  without  feeding. 
On  the  oak  some  feeding-  occurred  and  one  larva  went  through 
the  fourth  instar  in  eight  days  and  then  died,  starved,  in  fourteen. 
Three  of  the  others  died  of  starvation  in  four  and  one-half  days 
or  less,  and  one  was  accidentally  killed.  Although  alder  is  more 
closely  related  to  birch  than  is  oak,  yet  the  black  oak  was  preferred 
as  food,  though  it  could  not  sustain  the  larvae.  Ten  larvae  were 
then  similarly  kept  with  the  leaves  of  these  two  plants,  but  the 
leaves  were  previously  dipped  in  a  distillate  from  an  aqueous 
extract  of  birch  leaves.  It  is  sometimes  possible  to  make  insect 
larvae  eat  materials  that  have  the  odor  of  their  food  plants.  Of 
the  ten  larvae  used  in  this  case,  five  were  in  the  fourth  instar  and 
five  in  the  fifth.  On  the  alder  both  instars  fed  a  little.  One 
fifth-instar  larva  lived  ten  days,  and  two  fourth-instar  larvae  lived 
seven  days,  but  none  went  through  a  complete  instar.  On  the  oak 
there  was  more  feeding  than  on  the  alder.  One  fifth-instar  larva 
spun  a  cocoon  after  five  days,"  and  three  others  lived  between  seven 
and  nine  and  one-half  days.  One  fourth-instar  larva  lived  six- 
teen and  one-half  days,  molting  meanwhile,  and  three  others  lived 
between  five  and  one-half  and  eight  days.  In  only  one  instance 
on  the  oak  was  the  fourth  instar  completed.  Although  the  dis- 
tillate from  the  birch  extract  made  the  alder  and  oak  more  attrac- 
tive to  the  larvae,  and  they  ate  relatively  much  more  of  the  leaves 
wThen  so  treated,  they  did  not  show  any  growth  except  in  the  one 
instance  mentioned  above.  All  but  one  gradually  shrunk  in  size 
and  finally  died  of  starvation  before  molting.  Control  larvae  fed 
on  the  gray  birch  were  normal  in  development.  On  this  basis  the 
possibility  of  larvae  under  natural  conditions  living  on  either  oak 
or  alder  seems  remote,  and  the  reports  of  feeding  on  these  plants 
were  probably  cases  of  misidentification  of  the  insect  in  question. 

Under  laboratory  conditions  the  larvae  from  the  gray  birch  very 
readily  eat  leaves  of  paper  and  black  birch,  and  larvae  from  paper 
birch  just  as  readily  eat  leaves  of  gray  birch.  In  all  cases  the 
larvae  will  mature.  The  trials  conducted  were  not  sufficiently 
extensive  to  determine  whether  or  not  there  is  a  racial  difference 
in  the  individuals  from  different  host  plants.  This  racial  differ- 
ence would  be  primarily  manifested  by  the  oviposition  response 
of  the  adult,  and  difficulties  in  securing  eggs  consistently  from 
females  have  precluded  any  definite  experimental  evidence  on  this 
matter  to  date.  When  the  larvae  were  reared  in  the  laboratory 
they  were  placed  on  the  plants  under  trial,  and  if  they  left  these 
plants,  they  were  put  back  again.     This  was  continued  until  they 


448  CONNECTICUT    EXPERIMENT    STATION  BULLETIN    288 

ate  the  leaves  or  died.  Under  normal  circumstances,  no  such 
condition  would  be  met,  and  it  is  conceivable  that  the  larvae  might 
well  starve  to  death  in  the  midst  of  food  which  would  sustain 
life,  but  which,  for  various  reasons,  they  would  not  eat.  The 
preference  for  birch  as  food,  as  concerns  the  larvae,  is  partly  con- 
trolled by  a  chemical  sense,  for  they  eat  oak  and  alder  leaves  more 
readily  when  these  are  first  dipped  in  a  distillate  from  an  extract 
of  birch  leaves. 


•IX.     Factors  Affecting  Abundance 

The  phenomenon  of  periodic  outbreaks  of  Bucculatrix  has  been 
dealt  with  historically  in  previous  pages.  Some  of  the  factors 
which  have  a  bearing  on  the  abundance  and  rate  of  increase  of 
this  insect  deserve  consideration.  These  may  be  grouped  under 
food  supply,  climate,  and  natural  enemies  (including  diseases). 
Man  has  not  as  yet  played  any  direct  role  in  the  control  of  this 
species. 

There  is  no  scarcity  of  food  plants  in  the  northern  United  States 
and  southern  Canada,  and  the  endemic  population  of  Bucculatrix 
has  no  apparent  effect  on  the  growth  of  birch  trees.  Between 
outbreaks  the  larvae  are  scarcely  noticeable.  Paper  birch  forms 
a  great  part  of  the  subarctic  transcontinental  forest  and  is  a  very 
common  tree  as  far  south  as  the  Great  Lakes  and  central  New 
England.  Gray  birch  is  common  farther  south,  and  in  New  Eng- 
land and  New  York  it  is  a  weed  tree  which  is  constantly  encroach- 
ing on  cleared  land.  These  two  are  the  principal  food  plants 
and  neither  is  being  extensively  cut  by  man.  During  an  outbreak, 
when  the  larvae  frequently  eat  all  the  foliage  on  the  trees  over 
considerable  areas,  the  birches  are  not  killed,  even  by  several 
attacks  in  successive  years,  due  to  the  lateness  of  the  feeding 
period.  The  greater  amount  of  feeding  occurs  during  the  last  of 
August  and  September,  and  at  this  time  of  the  year  the  trees  have 
passed  through  the  most  active  season  and  are  not  so  severely 
injured  as  they  would  be  by  a  similar  attack  earlier  in  the  summer. 
This  insect  could  probably  never  eliminate  its  food  plant  in  any 
given  region.  It  very  probably  checks  the  growth  of  the  trees 
the  year  after  a  severe  attack,  but  this  check  would  not  be  suffi- 
ciently great  to  cause  a  decrease  in  the  available  larval  food  sup- 
ply. Another  factor  that  sometimes  has  some  effect  on  the 
abundance  of  a  particular  insect  is  the  competition  for  food  with 
other  species  of  insects.  The  defoliation  of  the  birches  in  any 
region  early  in  the  summer  would  very  obviously  affect  the  sur- 
vival of  Bucculatrix,  which  feeds  late  in  the  season.  At  present 
this  factor  cannot  be  considered  as  of  much  importance.  One  of 
the  most  serious  insect  enemies  of  the  birch  in  New  England  is 
the  saw-fly,  Fenusa  pumila  Klug,  whose  larvae  mine  the  leaves 
during  the  entire  summer,  as  there  are  several  generations.     Since 


BIOLOGY    OF    BIRCH    LEAF    SKELETONIZER  449 

this  insect  confines  its  work  entirely  to  the  new  terminal  growth, 
while  Bucculatrix  larvae  feed  by  preference  on  the  older  leaves 
of  the  tree,  the  two  live  together  in  harmony.  There  is  always 
the  possibility  of  the  last  Bucculatrix  larvae  of  the  brood  not  hav- 
ing sufficient  food,  because  of  the  work  of  the  earlier  developing 
part  of  the  brood,  and  hence  being  unable  to  survive.  The  habit 
of  spending"  two  days  in  a  quiescent  state  in  the  molting  web 
increases  this  danger,  for  during  these  two  days  the  foliage  on  the 
tree  may  be  entirely  consumed.  All  the  observations  made  in  the 
field  indicate,  however,  that  there  is  no  reason  to  believe  that  there 
occurs  any  decrease  in  food  supply  which  would  have  any  very 
important  effect  in  reducing  the  numbers  of  this  insect  even  fol- 
lowing a  year  when  it  was  abundant. 

No  data  have  been  obtained  on  the  effect  of  climate  on  the  sur- 
vival of  this  species.  The  greatest  danger  to  an  insect  is  during 
the  hibernating"  period,  when  severely  cold  weather  sometimes  kills 
off  much  of  the  population  of  certain  species.  It  is  a  well-known 
fact,  however,  that  insects  which  hibernate  under  the  snow  are 
better  able  to  survive  extremes  of  cold  than  species  which  hiber- 
nate above  the  snow  line.  For  this  reason  a  very  cold  winter 
would  not  be  expected  to  have  a  very  great  effect  on  the  popula- 
tion of  the  birch  Bucculatrix.  This  is  an  indigenous  insect  and 
is  inured  to  the  climate  of  its  present  geographical  range,  and  the 
greatest  effect  of  climate  on  its  abundance  is  probably  indirectly 
through  limitations  on  the  distribution  of  its  food  plants. 

It  is  not  inferred  that  climatic  variations  have  no  effect  on  the 
population,  but  rather  that  climate  alone  is  not  responsible  for  the 
more  or  less  regular  rise  and  fall  in  abundance. 

The  parasites  and  predaceous  enemies  of  this  insect  probably 
account  for  the  increase  and  decrease  in  its  numbers  more  than 
any  other  one  factor.  Ten  species  of  Ichneumonoidea  and  Chal- 
cidoidea  have  been  reared  from  the  larvae  and  pupae.  One  of 
these,  Hemiteles,  is  very  probably  hyper-parasitic,  as  Viereck 
(1916)  states  that  all  the  species  of  this  genus  are  probably  sec- 
ondary or  hyper-parasites.  The  10  species  with  the  stage  of  the 
host  from  which  they  emerged  are  listed  below : 

Stage 
of  Host 

1.  Bitccidatriplcx  sccundus  Viereck     Braconidae   pupa 

2.  Halticliclla  xanticle's  Walker     Chalcididae   pupa 

3.  Gelis  urbanus  Brues     Ichneumonidae   pupa 

4.  Cirrospilus  ocellatus  Girault     Elachertidae   larva  (ext.  feeding) 

5.  Gelis  bucculatricis  Ashmead     Ichneumonidae  pupa 

6.  Mesochorus  sp.     Ichneumonidae  pupa 

*/.  Pleurotropis  bucculatricis  Gahan     Entedontidae  ....  pupa 

8.  Closteroceru s  (cinctipcimis  Ashmead?)    Entedontidae  larva  (mining) 

9.  Dcrostcnus  sp.     Entedontidae    larva  (mining) 

10.  Hemiteles  sp.     Ichneumonidae pupa 

*  This  is  a  new  species  the  description   of  which,  by   Gahan,   is  published  in   Psyche, 
volume   34,   June,    iQ-'7. 


45°  CONNECTICUT    EXPERIMENT    STATION  BULLETIN    288 

The  family  names  are  those  used  by  Viereck  (1916).  These 
species  are  all  small  and  occur  singly  in  the  host.  The  extent  to 
which  they  parasitize  the  host  varies,  of  course,  from  year  to 
year  and  in  different  localities. 

In  the  winter  and  spring  of  1924  there  were  collected  397 
cocoons  from  which  there  were  secured  29  parasites  as  follows : 

Gelis  bucculatricis   14  specimens 

Bucculatriplex  secundus  7 

Haltichella  xanticles   6 

Hemiielcs    sp 2 

These  cocoons  were  collected  from  several  localities  around 
New  Haven,  where  the  host  had  been  abundant  in  1923.  A  large 
number  of  pupae  died  without  metamorphosing,  and  only  152 
adult  moths  were  secured  from  this  lot.  In  1925  most  of  the 
pupae  of  which  records  were  kept  were  from  larvae  reared  in  the 
insectary,  and  the  parasitism,  therefore,  was  abnormally  low. 
Nine  individuals  of  Bucculatriplex  secundus  and  two  of  Halti- 
chella xanticles  were  obtained  from  352  cocoons.  Conditions  dur- 
ing the  1925  season  of  emergence  were  not  normal,  as  the  cocoons 
had  to  be  kept  in  the  laboratory.  The  records  are  not  comparable 
to  those  obtained  a  year  later.  In  1926  there  were  collected  dur- 
ing April  and  May  209  cocoons  in  a  locality  where  the  larvae  had 
been  very  abundant  the  previous  season.  No  collections  had  been 
made  in  this  locality  during  either  1924  or  1925.  All  these 
cocoons  contained  pupae  (as  later  examination  showed),  and 
from  them  were  secured  53  parasites  and  58  adult  moths.  The 
cocoons  were  kept  outdoors  in  a  shaded  place  until  the  emergence 
period  was  passed,  and  then  those  from  which  no  insects  had 
emerged  (98  cocoons)  were  examined.  Five  contained  dead 
parasites  and  93  contained  dead  pupae.  Of  the  insects  which 
emerged,  then,  47.7  per  cent  were  parasites,  and  of  the  total  num- 
ber of  pupae  collected  27.7  per  cent  were  parasitized.  The  para- 
sites were  of  the  following  species : 

Bucculatriplex  secundus 37  specimens 

Pleurotropis  bucculatricis   12 

Haltichella  xanticles  1  specimen 

Gelis  urbanus 2  specimens 

Undetermined  (escaped)    .............  1  specimen 

It  is  evident  that  of  the  insects  which  emerge  from  the  cocoons 
the  parasites  make  up  a  large  percentage,  and  the  parasites  are 
better  able  to  survive  than  the  host.  Of  the  151  non-parasitized 
pupae,  only  58,  or  38.4  per  cent,  produced  adults,  whereas  of  the 
parasitized  pupae,  58  in  all,  53  or  91.4  per  cent  produced  parasites. 
The  presence  of  a  parasite  in  a  pupa  is  very  easy  to  determine 
after  three  months,  as  by  this  time  the  parasite  has  consumed  most 
of  the  host  tissue.     A  parasite  could  not  have  been  easily  over- 


BIOLOGY    OF    BIRCH    LEAF    SKELETONIZER  451 

looked  in  the  examination  of  dead  pupae.      Since  these  parasites 
occur  singly  in  the  host,  the  percentages  are  comparable.     There 
is,  of  course,  the  possibility  that  some  of  the  parasitized  pupae 
died  before  the  parasites  had  developed  far  enough  to  be  observed 
in  a  dead  and  desiccated  host.    The  fact  that  of  the  93  dead  pupae 
above  mentioned  44  had  reached  the  pre-imaginal  stage  before 
dying  indicates  that  this  possibility  would  have  no  great  bearing 
on  the  results  obtained,  for  had  any  parasite  been  present  in  any 
of  these,  it  would  have  prevented  the  host  from  reaching  the  con- 
dition of  the  pre-imago.     It  is  also  true  that  the  parasites  are  bet- 
ter able  to  withstand  high  and  low  temperatures  during  the  period 
of  emergence  than  is  the  host.     Three  lots  of  20  cocoons  each 
were  kept  at  different  temperatures,  one  at  3i-33°C,  one  at  room 
temperature  which  varied  between  180  and  260,  and  one  at  8-1 50. 
The  cocoons  were  placed  in  test  tubes  (50  cc.  capacity),  10  in  each 
tube.     To  serve  as  a  check  on  the  humidity  effect,  one  of  the  tubes 
of  each  lot  contained  a  piece  of  wet  blotting  paper  which  produced 
a  moisture-saturated  atmosphere  in  that  tube.     The  other  tube 
received   nothing.     All   tubes    were   kept   corked    except    for    an 
interval  of  about  one  minute  each  day  when  they  were  opened  in 
the  room  in  order  to  renew  the  air  supply.     The  relative  humid- 
ity of  the  room  averaged  67  per  cent,  with  a  variation  of  13-14 
per  cent  each  side  of  this  for  brief  intervals  of  time.     The  experi- 
ment began  June  2,  1926.     From  the  cocoons  held  at  room  tem- 
perature   18   insects   were   secured,    nine   from   each   tube.     This 
represented  a  normal  emergence.     Four  of  these  were  parasites, 
all  Biicculatriplex  secundus,   and   14  were  adult  moths.     From 
the    cocoons    held    at    8-1 5  °,    two    parasites    only    emerged,    one 
from  each  tube.     One  was  a  specimen  of  Biicculatriplex,  and  the 
other   was    Mesochorus   sp.     From   the   cocoons   held   at    31-330 
two    parasites    only    emerged,     both     from    the    tube    contain- 
ing room   air.      Both   were  Haltichella  xanticles.     After   being 
examined  July  10th,  all  the  cocoons  from  which  no  insects  had 
emerged  were  removed  to  the   outdoor   insectary.     Eight   adult 
moths  subsequently  emerged  from  the  tubes  that  had  been  held 
at   8-1 5°,    four    from    each    tube.     After    the    emergence    period 
was  well  passed  the  remaining  cocoons  were  examined.     No  dead 
parasites  were  found,  and  most  of  the  dead  pupae  had  reached  the 
pre-imaginal   stage.     Although  the  number  of   insects  concerned 
was  not  large,  the  parasites  were  very  evidently  better  able  to 
withstand  the  extremes  of  temperature  than  was  the  host,  for  all 
the  parasites  emerged  under  these  conditions,  but  no  moths  were 
obtained.     The  two  parasites  which  came  out  of  the  tubes  held 
at  8-1 50  emerged  July  3,  and  the  two  from  the  tubes  held  at  31- 
330  emerged  June  5  and  June  6  respectively.     In  the  case  of  the 
latter  two,  it  might  be  suspected  that  the  difference  in  develop- 
ment between  host  and  parasites  enabled  the  parasites  to  complete 


45 2  CONNECTICUT    EXPERIMENT    STATION  BULLETIN    288 

the  metamorphosis  and  emerge  when  the  host  could  not,  for  they 
were  exposed  to  the  high  temperature  only  three  and  four  days. 
However,  from  a  third  tube  set  up  the  same  as  the  others  but  con- 
taining calcium  chloride  and  held  likewise  at  31-330,  there 
emerged  two  parasites  only,  one  specimen  of  Bucculatriplex 
secundus  on  June  23,  and  one  specimen  of  Pleurotropis  buccula- 
tricis  on  July  4.  The  cocoons  in  this  last  tube  were  exposed  not 
only  to  the  high  temperature,  but  also  to  the  desiccating  effect  of 
the  chloride.  No  adult  moths  were  secured,  and  no  parasites  died 
before  emerging. 

In  addition  to  this  emergence  of  parasites  from  pupae,  there  is 
sometimes  a  considerable  parasitism  of  the  mining  larvae  by  Clos- 
teroccrus  and  Derostenus.  When  these  parasites  were  first  dis- 
covered, it  was  thought  that  they  were  one  and  the  same  species, 
as  they  were  in  the  larval  stag'e  and  resembled  each  other  closely. 
They  are  therefore  grouped  together  here.  If  the  mines  of  the 
Bucculatrix  larvae  are  examined  in  September,  many  will  be  seen 
to  contain  the  remains  of  the  larva  and  in  addition  a  very  minute 
parasite  larva  about  .75  mm.  in  length.  September  10  and  14, 
1925,  there  were  collected  619  Bucculatrix  mines  in  gray  birch 
leaves.  Of  these,  522  were  vacant  and  showed  by  the  exit  hole 
that  the  Bucculatrix  larva  had  emerged  normally.  The  other  97, 
or  15.7  per  cent,  contained  each  the  remains  of  a  Bucculatrix  larva 
and  one  parasite  larva  belonging  to  one  of  the  two  genera  in  ques- 
tion. The  only  exception  to  this  was  one  mine  which  contained 
two  parasite  larvae.  The  first  of  October,  1926,  the  same  locality 
was  visited  and  289  mines  were  collected.  The  mines  this  year 
were  much  less  abundant  than  in  1925.  Of  these  289  mines,  100 
had  been  normally  vacated  by  the  Bucculatrix  larvae  and  58,  or 
20.1  per  cent,  contained  parasites.  The  remainder,  131,  contained 
dead  Bucculatrix  larvae,  but  the  cause  of  their  death  could  not 
be  determined.  It  could  hardly  have  been  the  parasites  in  ques- 
tion, for  the  larz>ac  of  these  two  species  were  found  in  the  other 
mines. 

The  above  figures  show  that  there  may  be  a  heavy  mortality  of 
the  host  by  the  combined  attack  of  the  parasites.  Of  these,  Buc- 
culatriplex secundus  is  the  most  commonly  found.  Only  one 
locality  has  been  examined  for  Derostenus  and  Closterocerus,  and 
it  is  not  known  just  how  widely  spread  these  two  species  are. 
Pleuro'tropis  bucculatricis,  Haltichella  xanticles,  and  Gelis  buccu- 
latricis  are  also  rather  common.  One  specimen  only  has  been 
secured  of  Mesochorus  and  Cirrospilus  ocellatus.  The  former 
emerged  from  a  cocoon  in  1926,  and  the  latter  was  found  in  the 
pupal  stage  in  a  molting  web  of  Bucculatrix.  Hemiteles  may  be 
a  secondary  parasite  and  hence  of  no  use  in  checking  the  repro- 
duction of  Bucculatrix.  There  is  a  possibility  that  some  of  the 
others  also  are  secondary  parasites. 


BIOLOGY    OF    BIRCH    LEAF    SKELETONIZER  453 

The  adults  of  Derostenus  appear  the  last  of  the  summer,  but 
the  adults  of  the  other  parasites  appear  about  the  same  time  that 
the  host  adults  appear.  This  indicates  that  there  may  be  other 
hosts  for  some  of  the  parasites.  Bucculatrix  sccundus  hibernates 
as  a  larva  in  the  pupal  cuticle  of  the  host.  Derostenus  and  Clos- 
terocerus  kill  the  host  larva  before  it  completes  the  third  instar 
and  hibernate  as  larvae  in  the  mines  of  the  host.  The  other  spe- 
cies hibernate  in  the  pupal  cuticle  of  the  host,  but  the  hibernating 
stage  of  these  was  not  determined.  All  the  parasites  are  minute. 
The  Ichneumonoidea  adults  are  about  1.75-2.00  mm.  in  length, 
and  Haltichella  xauticlcs  is  about  the  same  size.  Plcurotropis 
buccidatricis  is  about  1.5  mm.  long;,  and  Derostenus  and  Clostero- 
cerus  are  each  about  .60  mm.  in  length. 

More  important  as  enemies  of  the  Bucculatrix  larvae  than  any 
one  of  the  above  species  of  parasites,  and  perhaps  than  all  of  them 
combined,  are  the  various  species  of  ants  and  other  predaceous 
insects  which  capture  the  larvae  when  they  descend  to  the  ground 
to  pupate.  Ants  will  not  only  capture  the  larvae  before  the  cocoon 
is  well  begun,  but  will  also  pull  a  larva  out  of  the  cocoon  in  which 
it  is  almost  entirely  enclosed.  In  1925  ants  destroyed  the  entire 
stock  of  larvae  in  the  insectary.  On  one  occasion  the  litter  on 
the  ground  under  a  birch  which  had  borne  hundreds  of  larvae 
was  very  carefully  examined  for  cocoons  after  all  the  larvae  had 
disappeared,  and  not  over  25  entire  cocoons  were  found.  A  large 
number  of  the  cocoons  were  partly  completed.  This  tree  had 
been  under  observation  and  no  extensive  mortality  of  the  larvae 
on  the  leaves  was  noticed.  There  is  no  question  that  most  of  the 
larvae  reached  the  ground,  and  most  of  these  fell  prey  to  their 
insect  enemies  before  they  could  pupate.  In  collecting  cocoons 
in  the  field  in  localities  where  there  has  been  an  outbreak  of  larvae 
and  the  trees  have  been  practically  defoliated,  it  is  surprising  to 
find  relatively  few  cocoons  that  are  entire  and  contain  pupae. 

Although  no  detailed  observations  have  been  made  on  the  activ- 
ities of  birds,  Dr.  Britton  informs  me  that  he  has  observed  certain 
warblers  apparently  feeding  extensively  on  the  larvae.  While 
there  is  no  question  that  birds  do  have  some  effect  on  the  abund- 
ance of  these  insects,  the  effect  of  ants  and  other  predaceous 
insects  seems  to  be  much  greater. 

The  interrelations  of  host,  parasites,  and  predaceous  foes  have 
been  very  clearly  described  in  the  case  of  the  fall  webworm  by 
Tothill  (1922),  whose  conclusions  are  here  briefly  summarized, 
and  many  of  the  reasons  for  the  occurrence  of  outbreaks 
and  the  following  decline  in  numbers  of  this  insect  are  applic- 
able to  Bucculatrix  canadensisella.  Under  normally  balanced 
natural  conditions  the  parasites  are  most  effective  and  keep  the 
host  in  an  endemic  and  harmless  state  for  a  number  of  years. 
The  predaceous  enemies  are  also  effective,  for  without  their  help 


454  CONNECTICUT    EXPERIMENT    STATION  BULLETIN    2CO 

the  host  might  increase  in  spite  of  the  parasites.  The  combined 
attack  of  parasites  and  predaceous  foes  reduces  the  numbers  of 
the  host,  but  at  the  same  time  the  number  of  parasites  is  reduced, 
for  a  competitive  struggle  for  food  occurs  among  the  species  of 
parasites  and  among  the  members  of  one  species.  During  the  last 
few  days  of  its  life  in  the  host,  the  parasite  is  much  more  destruc- 
tive to  the  host  tissue  than  at  any  other  time,  and  although  several 
parasites  may  start  life  in  one  host,  which  is  particularly  the  case 
when  the  host  becomes  scarce,  the  only  individual  that  survives 
is  the  one  which  first  reaches  this  rapidly  destructive  stage,  the 
others  perishing  from  lack  of  food.  The  predatory  enemies 
apparently  do  not  discriminate  in  favor  of  the  parasitized  larvae, 
and  this  also  tends  to  reduce  the  number  of  parasites.  Some 
species  of  parasites  may  become  locally  extinct,  and  not  being 
strong  fliers,  do  not  come  in  again  from  the  surrounding  territory 
for  some  years.  Any  environmental  change  favorable  to  the  host 
now  gives  it  an  opportunity  to  increase  in  the  absence  of  a  large 
part  of  its  enemies,  and  it  soon  reaches  a  stage  of  great  abundance. 
After  a  period  of  years  the  parasites,  which  have  now  found 
themselves  provided  with  an  abundant  food  supply,  increase,  and 
finally,  with  the  aid  of  the  predaceous  foes,  overcome  the  host 
and  again  reduce  its  numbers  to  an  endemic  state.  Over  a  long 
period  of  years  the  result  of  these  opposing  factors  is  a  series  of 
outbreaks  following  each  other  at  more  or  less  regular  intervals. 
When  the  host  begins  to  decrease  markedly,  the  parasites  also 
begin  to  decrease,  since  they  have  more  difficulty  in  finding  the 
host,  so  during  the  decline  of  the  host  population  there  is  not  nec- 
essarily an  increase  in  the  percentage  of  parasitism.  For  exam- 
ple, during  1925  the  parasitism  of  Bucculatrix  canadensisella 
mining  larvae  by  Closterocerus  and  Derostenus  was  15.7  per  cent, 
and  the  following  year,  in  the  presence  of  a  very  marked  reduc- 
tion in  the  abundance  of  mines,  the  parasitism  from  these  two 
species  was  increased  only  4.4  per  cent. 

When  the  larvae  of  Bucculatrix  are  abundant  there  may  be 
expected  up  to  20  per  cent  parasitism  in  the  mining  instars  and 
an  equal  percentage  of  parasitized  pupae.  To  this  must  be  added 
a  heavy  mortality  due  to  predaceous  enemies.  There  are  also 
certain  undetermined  factors,  possibly  both  internal  and  external, 
which  prevent  the  development  of  the  insect  beyond  the  pupal 
stage  and  cause  the  mortality  of  a  number  of  pupae.  These  last 
factors  are  more  effective  on  the  host  than  on  the  parasite.  Aside 
from  the  effect  of  parasites,  a  considerable  number  of  the  mining 
larvae  may  sometimes  succumb  from  some  cause  unknown  to  the 
writer.  All  the  factors  except  parasites  maintain  a  constant 
attack  on  the  various  stages  of  Bucculatrix  canadensisella,  and 
when  the  Bucculatrix  population  begins  to  decline,  the  severity  of 
this  attack  is  more  keenly  felt.     A  parasite  population  fluctuates 


BIOLOGY    OF    BIRCH    LEAF    SKELETONIZER  455 

with  a  host  population  and  has  direct  bearing  on  the  periodic 
abundance  of  the  host  but  cannot  entirely  eliminate  it,  as  the  para- 
sites decrease  when  the  host  decreases. 

A  species  of  fungus  belonging  to  the  genus  V  erticillium  has 
frequently  been  found  growing  on  the  dead  pupae  of  this  insect, 
and  it  was  thought  at  first  that  this  might  possibly  be  the  cause 
of  these  fatalities.  Several  attempts  to  inoculate  healthy  normal 
pupae  with  cultures  grown  on  oat  agar  failed  completely.  The 
procedure  followed  was  to  make  a  small  opening  in  the  cocoon 
and  expose  the  pupa  within.  A  drop  of  water  containing  a  sus- 
pension of  the  spores  and  mycelium  was  placed  on  the  pupa, 
which  was  then  set  aside  in  a  petri  dish  for  future  observation. 
Although  a  number  of  inoculations  were  made,  in  not  a  single  case 
did  an  infection  of  the  pupa  develop,  and  it  was  concluded  that 
the  fungus  concerned  is  entirely  saprophytic.  Several  species  of 
V  erticillium  are  found  on  dead  insects.  I  am  indebted  to  Dr. 
McCormick  of  the  Connecticut  Experiment  Station  for  determin- 
ing this  fungus  and  for  carrying  out  the  inoculations. 

X.     Geographical  Distribution 

This  insect,  as  far  as  reports  in  the  literature  and  information 
acquired  directly  from  entomologists  indicate,  is  found  only  in  the 
northern  United  States  and  in  Canada.  Its  southern  limit  is 
North  Carolina,  and  in  Canada  it  occurs  in  New  Brunswick,  Que- 
bec, Ontario,  Manitoba,  Saskatchewan,  Alberta,  and  British  Col- 
umbia. Mr.  Hutchings,  of  the  Entomological  Branch,  Ottawa, 
informs  me  that  it  probably  occurs  up  as  far  as  the  Yukon.  It 
is  recorded  as  far  west  as  Minnesota  in  the  United  States.  In 
Ontario,  Quebec,  New  Brunswick,  the  New  England  States,  New 
York,  Michigan.  Wisconsin,  and  Minnesota  it  is  very  common 
and  sometimes  appears  in  such  numbers  that  the  birches  are  defoli- 
ated. On  the  map  (figure  34)  is  marked  with  a  cross  every 
locality  from  which  I  have  definite  records  of  the  occurrence  of 
the  insect. 

According  to  data  obtained  from  Sargent's  "Silva  of  North 
America"  (1896),  the  four  native  food  plants  (the  paper,  gray, 
yellow,  and  black  birches)  of  Bucculatrix  canadensis ella  occur  over 
a  much  wider  area  than  that  from  which  the  insect  is  reported. 
The  region  occupied  by  these  birches  is  shaded  on  the  map.  The 
paper  birch  (Bctula  papyrifera)  is  very  widespread  and  is  a 
favorite  food  plant.  It  is  found  almost  everywhere  within  the 
shaded  region  on  the  map,  but  it  is  not  abundant  west  of  the  Rocky 
Mountains  nor  south  of  Minnesota,  Wisconsin,  Michigan,  and 
New  York.  The  red  birch  (Betula  nigra)  is  not  a  common  tree 
in  northern  United  States  and  I  have  no  records  of  its  being 
attacked  by  this  insect.     Its  range  extends  much   further  south 


45^  CONNECTICUT    EXPERIMENT    STATION  BULLETIN    288 


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Fig.  34.  Distribution  of  Bucculatrix  canadensisella  Chambers  and  its  food 
plants.  The  shaded  area  shows  the  distribution  of  the  gray,  paper,  yellow, 
and  black  birches.  The  crosses  indicate  localities  from  which  the  insect  has 
been  recorded. 


than  that  of  the  other  birches.  The  parts  of  North  America  in 
which  these  food  plants  (B.  populifolia,  B.  papyrifera,  B.  lutea, 
and  B.  lenta)  are  found  corresponds  very  closely  with  the  boreal 
and  transition  zones  as  outlined  by  C.  Hart  Merriam  (1898), 
except  for  the  Rocky  Mountain  region  of  the  United  States.     In 


BIOLOGY    OF    BIRCH    LEAF    SKELETONIZER  457 

Indiana,  which  is  just  south  of  the  transition  zone,  the  insects  are 
not  plentiful,  and  in  New  Jersey  they  are  reported  from  two  coun- 
ties, Essex  and  Morris,  both  in  the  northern  part  of  the  state,  and 
both  within  the  transition  zone.  In  North  Carolina  adults  have 
been  collected  in  Jackson  County.  Although  the  geographical 
distribution  of  the  birches  in  the  North  includes  Newfoundland, 
there  are  no  records  of  the  occurrence  of  B.  canadetisisella  on  that 
island.  It  is  possible  that  the  distribution  of  this  insect  coincides 
with  that  of  the  paper,  gray,  yellow,  and  black  birches,  but  the 
map  clearly  shows  that  it  is  most  commonly  found  in  the  region 
around  the  Great  Lakes  and  thence  east  to  the  Atlantic  Ocean. 

That  the  pupal  stage  can  withstand  low  temperatures  is  quite 
evident,  for  the  region  around  Port  Arthur,  Ontario,  and  the 
northern  shore  of  Lake  Superior  very  frequently  reaches  between 
-20°F.  and  -30°F.  Temperatures  would  not  interfere  with  the 
spread  of  the  insect  rather  far  north  in  western  Canada,  for  the 
isotherms  during  the  winter  run  in  a  curve  from  Quebec  south, 
and  then  north  through  Saskatchewan  and  Alberta,  making  the 
Dakotas,  the  northern  shore  of  Lake  Superior,  and  the  region  just 
north  of  it  much  colder  than  regions  directly  to  the  east  and  west, 
and  this  insect  is  frequently  very  abundant  along  the  northern 
shores  of  Lake  Superior.  The  fact  that  it  hibernates  on  the 
ground  under  leaves  and  under  the  winter  snow  also  enables  it 
to  endure  a  very  cold  climate.  It  would  not  be  surprising  if  an 
examination  of  white  birches  during  the  last  of  the  summer  in 
the  northern  limits  of  the  range  of  this  tree  would  reveal  the  pres- 
ence of  this  insect.  The  southern  limit  of  the  insect  is  also  very 
probably  the  southern  limit  of  its  food  plants.  This  is  a  very 
small  and  inconspicuous  moth,  and  unless  it  is  present  in  large 
numbers,  it  is  easily  overlooked.  As  the  larval  food  plants 
become  scattered  along  the  limits  of  their  geographical  range,  the 
insect  becomes  less  noticeable.  That  it  has  not  been  reported 
from  more  localities  is  not  surprising. 

During  seasons  when  it  is  not  very  abundant  over  any  great 
area,  the  infestations  of  B.  canadetisisella  are  often  spotted,  and  a 
small  group  of  birches  may  have  their  leaves  completely  skele- 
tonized, while  one  hundred  yards  away  the  leaves  of  others  are 
practically  unharmed.  This  is  in  all  probability  due  to  the  fact 
that  the  insect  flies  very  little  and  very  rarely  goes  beyond  the 
shelter  of  the  birch  trees. 

The  insect  has  probably  reached  its  present  geographic  range 
by  entirely  natural  means  of  spread,  for  its  habits  preclude  any 
great  distribution  by  human  agencies.  It  is  found  on  the  trees 
only  in  the  larval  state,  and  then  on  the  leaves  only.  If  birch  trees 
are  shipped  any  distance,  transportation  always  occurs  when  the 
tree  is  dormant  and  bears  no  leaves.  Birches  are  cut  after  the 
leaves  fall,  so  that  there  is  little  probability  of  cocoons  occurring 
on   cut   timber.     Early   records   of    forest-inhabiting   species   of 


45 8  CONNECTICUT    EXPERIMENT    STATION  BULLETIN    288 

insects  are  none  too  common  in  North  America,  and  B.  canaden- 
sis ella  was  probably  very  prevalent  over  the  entire  area  from 
which  it  has  been  reported  before  Chambers  described  it  in  1875. 
Even  today  it  attracts  no  attention  except  during  those  periods 
when  it  becomes  extraordinarily  abundant  and  defoliates  the  trees. 

XI.     Effect  of  Temperature  on  Development 

In  view  of  the  fact  that  temperature  seems  to  be  a  very  impor- 
tant factor  in  the  development  of  these  insects,  experiments  were 
carried  out  to  determine  the  effect  of  different  temperatures  on  the 
larva  during  the  period  when  it  was  feeding  externally  on  the 
leaf ;  that  is,  during  the  fourth  and  fifth  instars.  The  tempera- 
tures used  ranged  from  io°  to  35°C,  and  each  temperature  was 
held  as  nearly  constant  as  possible  under  the  conditions.  Those 
at  the  lower  end  of  the  range,  io°,  n°,  120,  were  obtained  by 
using  ice-boxes.  Incubators  were  used  for  250,  290,  340  and  350, 
and  an  incubator  was  cooled  with  ice  for  140  and  150  ;  200,  21  °  and 
220  were  laboratory  temperatures.  Observations  were  made  at 
8.00  a.  m.,  2.00  p.  m.,  and  10.00  p.  mv  or  as  close  as  possible  to  these 
hours,  each  day,  and  the  temperature  and  condition  of  the  larvae 
noted.  This  gives  a  possible  error  of  four  to  five  hours  in  the 
observations,  but  in  a  series  of  observations  this  error  tends  to  be 
compensated.  The  temperature  for  any  given  larval  stage  is  the 
average  of  all  the  readings,  and  the  charted  temperatures  are 
those  obtained  daily  by  averaging  the  three  temperatures  for  the 
day.  In  all  cases  a  fairly  constant  daily  temperature  was  held. 
The  temperatures  were  averaged  for  each  individual  larva,  and 
the  fluctuations  of  a  degree  in  either  direction  made  the  mean 
temperatures  for  different  larvae  kept  in  the  same  location  vary 
slightly.  For  this  reason  the  groups  tabulated  under  11  °  and  120 
were  both  held  at  the  temperature  charted  on  line  (c),  text  figure 
35,  those  tabulated  under  140  and  150  were  held  at  the  temperature 
charted  on  line  (d),  those  under  200,  210,  and  220  on  line  (e), 
and  those  under  340  and  35 °  on  line  (h).  In  some  stages  there  is 
a  difference  of  only  one  degree  between  groups  with  very  little 
difference  in  the  duration  of  the  stage  at  the  different  tempera- 
tures. This  is  due  not  only  to  the  fact  that  one  degree  would  not 
be  expected  to  show  much  difference  under  the  conditions,  but 
also  to  the  method  of  averaging  temperature  readings.  The  tabu- 
lated temperatures  are  correct  within  one-half  of  one  degree ;  that 
is,  if  the  temperature  for  one  larva  during  the  fourth  instar  aver- 
aged 20.4°C,  that  larva  was  placed  in  the  20 °  group.  A  differ- 
ence of  one-tenth  of  a  degree  in  the  average  might  throw  the 
larva  into  a  higher  group,  for  if  the  temperature  averaged  20.5 °C, 
the  larva  was  placed  in  the  21  °  group. 


BIOLOGY    OF    BIRCH    LEAF    SKELETONIZFR  459 

In  view  of  the  fact  that  the  larvae  kept  in  ice-boxes  were  in 
darkness,  eight  larvae  wrere  reared  in  a  dark  box  at  the  laboratory 
room  temperature,  20°-2i°C,  as  a  check  on  the  effect  of  absence 
of  light.  Two  of  these  died  in  the  fifth  instar  and  the  other  six 
pupated.  The  mortality  was  not  exceptional.  The  mean  dura- 
tion of  the  fourth  instar  was  125  ±  5.1  hours,  with  a  standard 
deviation  of  21  hours;  the  mean  duration  of  the  fifth  instar  was 
131  ±  4.3  hours  with  a  standard  deviation  of  18  hours,  and  the 
mean  duration  of  the  entire  external  feeding  period  was  248  ±  6.8 
hours  with  a  standard  deviation  of  25  hours.  These  periods  are 
practically  the  same  as  the  periods  of  larvae  reared  at  the  same 
temperature  in  the  lighted  laboratory,  and  the  absence  of  light 
caused  no  error. 

The  larvae  were  reared  in  individual  glass  vials  as  described 
previously,  and  the  relative  humidity  was  kept  constant  by  wet 
sand  in  the  vials.  The  leaves  used  as  food  were  renewed  as  often 
as  was  necessary  for  keeping  the  food  material  fresh  and  unwilted. 
At  temperatures  of  25 °  and  higher,  the  leaves  were  renewed 
daily;  at  140,  150,  200,  21  °  and  220,  every  other  day;  at  all  temper- 
atures below  1 40,  twice  a  week.  The  leaves  used  were  all  from 
the  lower  parts  of  gray  birch  trees,  that  is,  the  older  leaves,  and  in 
all  but  three  or  four  instances  were  from  the  same  group  of  trees. 
Leaves  selected  were  as  uniform  as  possible.  All  the  larvae  used 
in  this  experiment  were  obtained  from  gray  birches  bordering  a 
field  about  eight  miles  north  of  New  Haven.  The  large  number 
of  larvae  reared  at  room  temperature  (200  and  21  °)  was  due  to 
the  fact  that  a  control  of  each  lot  of  larvae  was  kept  at  this 
temperature. 

Four  larval  periods  were  considered  :  ( 1 )  the  quiescent  period 
in  the  second  molting  web;  (2)  the  fourth  larval  instar,  which 
includes  the  period  in  the  second  molting  web  ;  (3)  the  fifth  larval 
instar  up  to  the  spinning  of  the  cocoon;  (4)  the  entire  period  of 
life  spent  outside  the  mine,  which  includes  the  fourth  and  fifth 
instars.  Although  the  fifth  instar  really  includes  a  prepupal 
period  in  the  cocoon,  observations  on  this  period  were  not  possible 
without  disturbing  the  conditions  of  the  experiment,  so  this  pre- 
pupal period  was  omitted.  The  actively  growing  period  of  the 
larva  is  over  wrhen  feeding  ceases,  and  the  omission  of  the  pre- 
pupal period  does  not  affect  the  results.  In  each  case  the  end- 
point  is  sharply  defined. 

The  chart  on  page  465,  figure  35,  gives  the  temperatures  at 
which  the  different  groups  of  larvae  were  kept,  and  the  letter  in 
parentheses  at  the  left  of  each  temperature  curve  corresponds  to 
the  same  letter  opposite  each  temperature  in  tables  4  to  7  and 
indicates  the  curve  for  that  temperature  group.  In  the  tables 
are  given  the  number  of  each  larva,  the  day  it  began  the  period 


460  CONNECTICUT    EXPERIMENT    STATION  BULLETIN    288 

represented  by  the  table,  the  duration  of  that  period,  the  mean 
duration  for  each  temperature  group,  and  the  standard  deviation 
for  each  temperature  group  (in  parentheses  after  the  mean).  The 
temperature  at  which  any  larva  or  group  was  held,  together  with 
the  temperature  fluctuations  during  the  period,  may  be  ascertained 
by  examining  the  temperature  chart  (figure  35).  Fluctuations 
occurred  one  degree  each  side  of  the  mean  except  in  a  few  cases 
where  a  brief  fluctuation  of  two  degrees  is  found.  The  latter 
cases  were  so  few  and  the  variations  in  temperature  for  any  one 
larva  were  of  such  brief  duration  that  the  results  are  not  affected. 
Table  8  on  page  481  is  a  condensation  of  the  other  tables  and  gives 
the  data  which  form  the  basis  for  the  curves  shown  in  figures  36 
to  40.  These  curves  show  the  relation  between  temperature  and 
development. 

Each  figure  contains  two  curves.  The  curve  marked  A  gives 
the  duration  of  the  period  in  hours  for  each  temperature  within 
the  limits  of  the  curve.  The  abscissae  represent  degrees  centi- 
grade, and  the  ordinates,  on  the  left  of  the  figure,  hours.  The 
number  of  degree-hours  (developmental  units)  required  for 
development  at  any  temperature  may  be  calculated  from  this  curve 
by  multiplying  time  by  temperature.  If  the  curve  conformed  to 
the  formula  of  a  true  equilateral  hyperbola,  the  number  of  degree- 
hours  for  each  point  on  the  curve  would  be  the  same,  according 
to  the  mathematical  definition  of  the  curve,  and  this  constant 
figure  would  be  the  so-called  "thermal  constant."  In  no  two 
consecutive  temperatures  of  the  experiment  were  the  number  of 
degree-hours  equal  or  approximately  equal,  and  the  curves  clearly 
show  that  no  thermal  constant  exists  in  the  development  of  the 
larvae  under  these  experimental  conditions.  The  curve  marked 
B  gives  the  index  of  development  for  each  degree  of  temperature. 
The  abscissae  are  the  same  as  those  of  the  A  curve,  and  the  ordi- 
nates, on  the  right  of  the  figure,  are  the  reciprocals  of  the  ordi- 
nates of  the  A  curve.  Each  point  on  the  B  curve  gives  that 
fraction  of  the  total  development  which  is  completed  in  one  hour 
at  that  particular  temperature.  The  curve  thus  gives  the  rate  of 
development  directly  and  changes  in  that  rate  corresponding  to 
changes  in  temperature.  If  curve  A  conformed  to  the  formula 
of  an  equilateral  hyperbola,  the  curve  B  corresponding  to  it  would 
be  a  straight  line  by  definition,  but  A  is  not  an  equilateral  hyper- 
bola, and  B  is  not  a  straight  line.  Where  a  thermal  constant 
exists,  the  rate  of  development  curve  B  is  always  rectilinear,  and 
if  a  thermal  constant  exists  for  any  narrow  range  of  temperatures, 
within  that  range  the  developmental  curve  is  straight.  Accord- 
ing to  the  data  and  the  curves,  there  is  no  thermal  constant  over 
any  range  of  temperatures  greater  than  the  error  of  the  experi- 
ment. In  drawing  the  curves,  the  points  were  plotted  for  the  A 
curve  from  the  experimental  data,  and  the  curve  was  made  to 


BIOLOGY    OF    BIRCH    LEAF    SKELETONIZER  46 1 

conform  to  these  points  as  closely  as  possible.  These  points  are 
enclosed  in  small  circles  in  the  figures.  The  index  of  development 
curve  B  was  then  drawn  to  conform  to  curve  A,  and  the  recipro- 
cals of  the  plotted  points  in  A  are  enclosed  in  circles  in  B.  In  the 
absence  of  a  thermal  constant,  the  rate  of  development  of  the 
larvae  and  the  degree-hours  required  for  the  completion  of  any 
stage  must  be  calculated  from  the  curves  directly.  No  attempt 
has  been  made  to  project  the  curves  beyond  the  limits  of  the 
experimental  data. 

The  lowest  constant  temperature  at  which  larvae  would  survive 
the  fourth  and  fifth  instars  and  pupate  was  found  to  be  between 
io°  and  I2°C,  and  the  highest  temperature  was  found  to  be 
slightly  under  34°C.  Eight  larvae  in  the  first  molting  web  were 
held  at  6°C.  [line  (a)]  6  days,  during  which  time  they  did  not 
molt,  and  four  larvae  were  held  at  3°-6°C.  [line  (a)]  10  days, 
during  which  time  no  molting  occurred.  The  duration  of  the 
period  in  the  molting  web  at  21  °  is  about  40  hours.  All  twelve  of 
these  larvae  molted  within  29  hours  after  removal  to  the  labora- 
tory, where  the  temperature  was  2o°-2i°  [line  (e)].  Develop- 
ment thereafter  was  normal.  Six  larvae  in  the  first  molting  web 
were  kept  at  9°-io°C.  [line  (b)].  None  of  these  completed 
development,  but  three  molted  within  9  days  and  lived  50  days.  39 
days,  and  10  days  respectively,  after  the  molt.  The  other  three  died 
in  the  web  without  molting.  Ten  larvae  in  the  second  molting 
web  were  kept  at  9°-io°C.  [line  (b)],  and  all  molted  within  8  days. 
Four  died  in  24-45  days,  and  six  pupated  in  18-30  days.  Controls 
of  all  these  larvae  kept  at  2o°-2i°  in  the  laboratory  were  normal 
and  had  a  mortality  of  zero  (25  larvae  in  all).  It  is  quite  appar- 
ent that  although  some  development  occurs  at  9°-io°C,  the  con- 
tinuous exposure  of  larvae  in  the  fourth  instar  to  this  temperature 
is  fatal.  If  the  larvae  are  in  the  fifth  instar,  they  may  or  may  not 
complete  development,  depending  on  individual  hardiness.  Ten 
larvae  in  the  first  molting  web  were  kept  at  n°-i2°C.  [line  (c)]. 
All  molted  within  4  days,  8  completed  the  fourth  instar,  and  7 
pupated.  Ten  larvae  in  the  second  molt  web  were  held  at  the 
same  temperature.  Eight  of  these  molted  within  3  days  and 
pupated.  Controls  of  these  two  lots  were  kept  at  20°-2i°  in  the 
laboratory  [line  (e)],  were  normal  in  development,  and  had  a 
mortality  of  2  larvae  in  22.  At  H°-I2°C.,  development  is  slow 
but  otherwise  is  normal.  It  may  possibly  be  that  the  fifth-instar 
larvae  are  more  able  to  withstand  low  temperatures  than  the 
fourth.  Death  at  9°-io°C.  seemed  due  to  starvation,  the  cold 
preventing  the  larvae  from  feeding,  and  even  under  normal  con- 
ditions the  fifth-instar  larvae  eat  more  constantly  than  do  those  of 
the  fourth  instar.  The  larvae  at  this  low  temperature  were  always 
sensitive  to  touch,  and  sometimes  spun  silk  threads.  It  appears 
as  if  the  first  effect  of  the  cold  was  to  stop  feeding  activity.     The 


462  CONNECTICUT    EXPERIMENT    STATION  BULLETIN    288 

larvae  then  shrunk  in  size  and  finally  died  from  lack  of  nourish- 
ment. The  curves  (figure  39)  of  development  for  the  period 
including  the  fourth  and  fifth  instars  necessarily  begin  at  I2°C, 
in  accordance  with  the  above  facts. 

At  the  high  temperatures  (34°-35°)  the  effect  was  somewhat 
different.  The  procedure  was  essentially  similar  to  that  described 
for  the  low  temperatures,  and  the  temperature  line  (h)  in  figure 
35  depicts  the  temperature  used.  This  temperature  was  also  fatal 
to  fourth-instar  larvae  if  maintained  continuously,  but  the  larvae 
were  able  to  molt,  complete  the  fourth  instar,  molt  again  and 
begin  the  fifth  instar.  None  pupated,  however.  Feeding  was 
very  actively  carried  on  until  about  12-24  hours  before  death,  and 
starvation  was  not  a  factor  to  be  considered.  Fifth-instar  larvae 
molted  and  pupated  without  difficulty.  The  data  for  the  larvae 
at  these  temperatures  is  given  in  the  tables.  Since  the  total  feed- 
ing period  could  not  be  completed  at  34°-35°C,  the  curve  of  this 
period  stops  at  2g°C,  the  highest  temperature  at  which  the  larvae 
completed  two  instars  and  pupated.  At  the  high  temperature,  as 
well  as  at  the  low,  the  fifth-instar  larvae  seemed  better  able  to 
endure  the  adverse  conditions  and  fed  more  consistently.  The 
lethal  high  temperature,  using  the  curve  as  a  guide,  is  in  all  prob- 
ability very  close  to  34°C. 

The  above  data  demonstrate  that  a  real  threshold  of  develop- 
ment ("developmental  zero")  and  a  real  maximum  lethal  tempera- 
ture are  determinable  only  when  the  length  of  exposure  to  those 
temperatures  is  considered,  and  that  some  development  may  occur 
at  temperatures  beyond  the  lethal  high  and  low  (for  the  entire 
period)  if  these  exposures  are  not  too  prolonged.  The  difference 
between  that  temperature  which  gives  a  maximum  speed  of  devel- 
opment and  the  lethal  high  is  much  less  than  the  difference 
between  the  same  maximum  and  the  lethal  low,  but  development 
occurs  at  both  extremes.  For  example,  at  9°-io°C,  the  fourth- 
instar  larvae  will  ultimately  die,  but  some  development  will  have 
occurred  meanwhile.  The  same  is  true  of  34°-35°  and  even 
higher,  for  in  another  experiment  an  exposure  to  370 C.  for  a  few 
hours  caused  neither  cessation  of  feeding  nor  other  deleterious 
effects.  In  calculating  the  effect  of  low  temperatures  on  the  life 
cycle  of  insects,  it  has  been  customary  to  eliminate  all  "ineffective 
temperatures" ;  that  is,  all  temperatures  below  a  given  threshold, 
this  threshold  depending,  of  course,  on  the  insect  in  question. 
Theoretically  the  developmental  curve  B  (figures  36  to  39)  should 
cut  the  temperature  axis  at  the  threshold  of  development.  The 
corresponding  theoretical  point  on  the  time-temperature  curve  A 
would  be  at  infinity.  The  curve  B,  however,  shows  no  marked 
inclination  toward  the  temperature  axis  at  its  lower  end,  and  it 
would  be  rash  to  predict  from  any  data  obtained  by  a  time-devel- 
opment study  just  where  it  is  going  to  cut  that  axis.  Under 
natural  conditions  the  temperature  fluctuates   considerably,   and 


BIOLOGY    OF    BIRCH    LEAF    S'KELETONIZER  463 

even  though  it  should  rise  above  or  sink  below  that  point  at  which 
the  larvae  could  not  complete  development,  some  development 
would  probably  take  place  at  these  extremes  of  the  fluctuations. 
The  fact  that  the  curve  A  is  not  a  true  equilateral  hyperbola  and 
the  curve  B  is  not  a  straight  line  makes  it  inadvisable  to  project 
these  curves  beyond  the  experimental  data  in  order  to  determine 
theoretical  points,  and  no  attempt  has  been  made  to  determine  an 
absolute  threshold  of  development. 

The  curves  A  and  B  in  figures  36  to  39  show  clearly  the  depress- 
ing" effect  of  high  temperatures  on  the  larvae.  According  to  these 
figures  the  maximum  rate  of  development  would  occur  at  300- 
3i°C,  and  the  experimental  data  give  the  maximum  rate  at  29°C, 
no  experiments  being  carried  out  between  29°C.  and  34°C. 
Either  side  of  the  30°-3i°  point,  the  rate  of  development  is  slower, 
whether  the  temperature  rises  or  falls. 

The  curve  B  shows  the  rate  at  which  the  speed  of  development 
varies  and  the  direction  of  this  variation  for  each  degree  of 
temperature.  Beginning  at  the  lowest  point  in  the  curve,  the  rate 
at  which  development  is  speeded  up  increases  with  each  increase 
in  temperature  until  at  a  certain  point,  the  steepest  part  of  the 
curve,  a  maximum  is  reached.  At  this  point  fluctuations  in 
temperature  have  their  greatest  effect  on  the  development  of  the 
larvae.  As  the  temperature  increases  above  this  point,  the  rate  at 
which  development  is  speeded  up  with  the  rising  temperature 
decreases  until  the  point  of  maximum  rate  of  development  is 
reached,  30°-3i°C.  Any  increase  in  temperature  beyond  this 
causes  an  actual  decrease  in  the  rate  of  development.  It  is  this 
variation  in  rate  of  development  which  forbids  the  "summing" 
temperatures  on  a    "developmental  unit"    basis. 

In  all  four  figures  (36  to  39)  the  curves  are  sigmoid.  In  the 
curves  representing  the  total  external  feeding  period,  those  tem- 
peratures beyond  I2°C.  in  one  direction  and  29°C.  in  the  other 
are  omitted,  due  to  the  non-survival  of  larvae  beyond  these  points, 
but  the  curves  take  the  same  general  form  as  the  others.  The 
effect  of  temperature  in  these  experiments  was  similar  for  both 
quiescent  and  active  periods.  The  shorter  periods  give  the  better 
developmental  curves,  for  as  the  period  lengthens  the  index  of 
development  becomes  less  numerically,  and  the  curve  "flattens," 
if  the  system  of  coordinates  remains  unchanged.  In  figure  40 
the  rates  of  development  as  actually  obtained  have  been  plotted 
on  logarithmic  paper,  and  the  curves  give  a  correct  impression  of 
the  relative  effects  of  temperature  for  the  different  stages.  The 
curves  are  much  more  nearly  parallel  than  those  plotted  on  arith- 
metic paper  and  show  that  the  effect  of  temperature  on  the  rate  of 
development  of  the  different  stages  is  somewhat  similar. 

Due  to  the  small  scale  on  which  the  curves  are  drawn,  the  steep- 
est part  of  any  one  of  the  curves  is  somewhat  difficult  to  determine 
by  mere  examination.     However,  a  calculation  of  the  tangents  of 


464  CONNECTICUT    EXPERIMENT    STATION  BULLETIN    288 

the  curves  at  all  temperatures  shows  that  for  the  period  in  the 
second  molting  web  this  steepest  part  lies  between  21  °  and  22°; 
for  the  fourth  instar,  between  230  and  240 ;  for  the  fifth  instar, 
between  240  and  250 ;  and  for  the  total  external  feeding  period, 
between  23 °  and  240.  The  tangents  of  these  parts  of  the  curves 
are  1.8855,  -6335,  .4280,  and  .2300  respectively.  These  tangents 
are  calculated  for  the  rate  curves  as  drawn.  The  lower  tempera- 
ture at  which  this  is  found  in  the  period  spent  in  the  second  molt- 
ing web  may  be  due  to  the  inactivity  of  the  insect.  The  tempera- 
ture relations  for  the  two  feeding  instars  approximate  each  other 
fairly  well. 

A  theoretical  use  of  such  curves  as  those  marked  B  in  the 
figures  in  considering  the  development  of  an  insect  under  natural 
conditions  where  fluctuating  temperatures  occur  is  in  the  predic- 
tion of  the  time  taken  to  complete  a  stage  of  development.  In 
making  such  calculations,  if  the  mean  temperature  of  a  short  dura- 
tion of  time,  a  few  hours,  for  example,  be  ascertained  and  the 
index  of  development  at  this  temperature  be  multiplied  by  the 
number  of  hours  during  which  this  mean  temperature  is  con- 
sidered effective,  the  amount  of  development  completed  during 
this  time  can  be  approximately  determined.  When  the  sum  of 
these  last  determinations  equals  1,  the  development  is  theoretically 
completed.  This  is  the  method  proposed  by  Sanderson  (1908) 
and  seems  to  be  more  logical  than  any  alternative  method.  In 
practice  it  has  been  customary  to  determine  the  mean  temperature 
for  each  hour.  The  method  more  commonly  used  in  determining 
amount  of  development  during  short  intervals  of  time  under  con- 
ditions of  fluctuating  temperatures  depends  on  the  assumption 
that  the  changes  in  velocity  of  development  vary  directly  with 
increases  in  temperature  and  that  the  velocity  curve  (B  in  the 
figures  in  this  paper)  is  a  straight  line  for  a  certain  temperature 
range,  if  not  for  all  temperatures  between  the  "threshold"  and 
the  "maximum."  With  such  an  assumption  there  exists  a 
thermal  constant  within  certain  temperature  limits  ("medial" 
temperature  according  to  Shelford — 1926)  and  the  number  of 
developmental  units  required  to  complete  development  is  this  ther- 
mal constant,  i.  e.,  the  product  of  time  by  temperature.  If  the 
amount  of  development  completed  during  a  brief  interval  of  time 
be  computed,  it  will  represent  a  fraction  of  the  thermal  constant 
{not  of  1),  that  is,  a  certain  number  of  developmental  units. 
When  the  sum  of  these  determinations,  the  total  of  developmental 
units,  equals  the  thermal  constant,  development  is  theoretically 
completed.  The  fault  with  this  method  lies  in  its  uselessness 
when  the  velocity  curve  is  not  a  straight  line,  and  even  if  part  of 
this  curve  is  assumed  to  be  straight,  the  method  is  not  good  for 
fluctuations  outside  of  this  temperature  range.  The  small  scale 
on  which  curves  are  plotted  sometimes  gives  an  erroneous  impres- 
sion of  rectilinearity. 


BIOLOGY    OF    BIRCH    LEAF    SKELETONIZER 


465 


466  CONNECTICUT    EXPERIMENT    STATION  BULLETIN    288 

Any  method  of  estimating  insect  development  by  averaging 
widely  fluctuating  temperatures  has  its  faults,  because  the  devel- 
opment of  an  insect  at  a  constant  temperature  is  not  uniform 
(Crozier  1926).  An  approximation  is  the  best  that  can  be 
attained. 

Although  it  is  not  the  purpose  of  this  paper  to  enter  into  a 
detailed  discussion  of  the  effects  of  temperature  on  insect  life,  a 
comparison  of  the  results  here  obtained  with  those  of  some  others 
is  of  interest.  For  further  information  on  the  subject  the  works 
of  Bachmetjew,  Sanderson  and  Peairs,  Headlee,  Krogh,  Glenn, 
Peirce,  Shelford,  Payne,  and  Crozier  may  be  consulted. 

Glenn  (1922)  has  attempted  to  show  that  in  the  case  of  the 
pupae  of  the  codling  moth  the  rate  of  development  curve  becomes 
a  straight  line  when  all  the  temperatures  below  a  physiological 
zero  (in  this  case  52 °F.)  are  eliminated  from  the  calculations  of 
the  mean  temperatures  and  suitable  corrections  are  made  for  the 
high  fluctuations  above  the  point  of  maximal  rate  of  development. 
The  data  were  obtained  under  field  conditions,  and  the  tempera- 
tures are  therefore  averages  of  daily  fluctuations.  However,  if 
the  calculations  are  made  of  the  values  the  lower  points  (5.10, 
6.8°,  7.0 °,  and  8.6°  effective  temperatures)  should  have  in  order 
to  fall  into  the  corrected  curve,  it  is  found  that  in  every  case  the 
value  is  lower  than  the  actual  data  give.  The  differences  are 
small,  but  the  error  probably  lies  in  the  fact  that  some  development 
actually  occurred  during  some  of  the  fluctuations  below  52°F., 
and  the  elimination  of  these  low  temperatures  from  the  calcula- 
tions caused  a  slight  deviation  from  the  expected  results. 

Recently  Shelford  (1926)  has  used  the  data  of  Krogh  (1914) 
on  the  development  of  the  pupa  of  Tenebrio  molitor  to  demon- 
strate the  fact  that  within  a  certain  temperature  range  the  rate  of 
development  curve  may  be  rectilinear  and  a  thermal  constant  may 
be  used.  In  this  particular  case  the  range  was  between  18.5 °C. 
and  28°C.  Crozier  (1926)  has  shown  that  the  simple  fact  that 
an  animal  has  passed  a  fraction  of  its  developmental  time  at  a  cer- 
tain temperature  does  not  necessarily  mean  that  that  particular 
fraction  of  its  development  has  been  completed,  for  the  rate  of 
development  is  not  uniform  at  any  one  temperature  for  the  entire 
duration  of  the  period  in  question,  and  the  curves  of  development 
at  any  two  temperatures  are  not  superimposable.  Thus,  if  one- 
half  of  the  developmental  period  is  passed  at  a  given  temperature, 
more  or  less  than  half  the  development  is  completed,  and  when  a 
shift  is  made  to  another  temperature,  there  remains  more  or  less 
than  one-half  of  the  development  to  be  completed  at  the  new 
temperature.  Since  the  curves  of  development  at  the  two  tem- 
peratures are  not  superimposable,  the  duration  of  the  total  period 
at  the  two  temperatures  would  depend,  among  other  things,  upon 
which  of  the  two  the  animals  were  first  exposed  to.     This  fact 


BIOLOGY    OF    BIRCH    LEAF    SKELETONIZER 


467 


Fig.  36.     Effect  of  temperature  on  the  period  in  the  second  molting  web. 
A  is  the  time-temperature  curve,  and  B  is  the  rate  of  development  curve. 


alone  would  throw  doubt  on  the  justification  of  using  a  thermal 
constant  for  any  range  of  temperatures.  Moreover,  the  actual 
data  of  Krogh  show  that  the  curve  is  not  truly  rectilinear  even 
between  18.5 °  and  280,  but  is  slightly  sigmoid.  The  calculations 
were  made  from  Krogh's  data,  and  the  last  figures  are  added  to 
show  the  trend  of  the  curve  outside  the  range  in  question. 

Temperature 

18.0  -20.90  C. 

20.9  -23.65 
23.65-27.25 
27.25-32.7 

If  the  curve  were  rectilinear  between  18.5 °  and  280,  the  second 
and  third  tangents  at  least  of  those  given  above  should  be  equal. 
Moreover,  according  to  Krogh's  own  statement  the  O  10  of  the 


Tangent  to  curve 

Angle  of  curve 

.8143 

39°     9 

.8291 

39     40 

.8500 

40    22 

•5321 

28       1 

4^8  CONNECTICUT    EXPERIMENT    STATION  BULLETIN    288 

Van't  Hoff  formula  does  not  hold  for  the  relation  of  temperature 
to  development  in  this  particular  experiment.  Most  curves  show- 
ing this  relation  have  the  curvilinear  form,  and  the  assumption  of 
rectilinearity  for  any  part  of  such  curves  is  hardly  justifiable. 

In  attempting  to  express  the  relation  of  temperature  to  develop- 
ment Crozier  and  others  have  brought  into  use  the  critical  thermal 
increment  of  the  Arrhenius  formula.     This  formula  is  as  follows : 


H l 

o  \  T 

K-2  ==   K-ie 


2\T,  T. 


Kx  is  the  rate  of  development  at  the  absolute  temperature  T1}  and 
K2  that  at  T2 ;  e  is  the  base  of  the  natural  system  of  logarithms, 
and  2  is  the  gas  constant,  /x  is  the  temperature  characteristic 
expressing  the  critical  thermal  increment.  It  has  some  theoreti- 
cal significance  because  it  expresses  the  heat  change  accompanying 
the  conversion  of  the  participating  molecules  in  the  reaction  from 
an  "inactive"  to  an  "active"  state,  and  hence  corresponds  to 
the  sum  of  the  heats  of  dissociation  of  the  substances  taking  part 
in  the  reaction.  The  formula  gives  consistent  results  for  catalytic 
reactions  in  pure  solutions,  and  the  value  of  //.  is  constant  over  a 
wide  range  of  temperature. 

Blackmail  (1905)  put  forward  the  suggestion  that  in  enzymatic 
reactions  such  as  characterize  biological  phenomena,  the  pace  of 
the  entire  reaction  is  governed  by  that  of  the  slowest  reaction  of 
the  series  composing  it,  and  it  is  this  principle  of  catenary  reac- 
tions being  controlled,  as  regards  their  velocity,  by  the  slowest  of 
the  catenary  series  that  Crozier  has  developed  and  used  in  explain- 
ing the  relations  of  temperature  to  growth  as  well  as  to  other 
biological  processes.  Within  a  certain  range  of  temperatures  a 
certain  reaction  may  be  the  slowest  in  the  process  and  hence  will 
govern  the  speed  of  the  whole,  but  when  the  temperature  rises 
above  a  definite  point,  another  reaction  of  the  chain  becomes  the 
slowest  and  hence  the  governing  one.  Within  the  temperature 
range  governed  by  one  reaction  of  the  series,  the  temperature 
characteristic  for  that  reaction  is  the  temperature  characteristic 
for  the  whole  process,  and  when  the  second  reaction  becomes  the 
governing  one,  a  different  characteristic,  that  of  this  second  reac- 
tion, governs.  In  a  monomolecular  reaction  the  curve  obtained 
by  plotting  the  logarithm  of  the  rate  against  the  reciprocal  of  the 
absolute  temperature  is  rectilinear,  as  can  be  readily  seen  from  the 
formula  of  Arrhenius.  The  problem  becomes  one  of  getting 
the  value  of  the  critical  thermal  increment  (temperature  charac- 
teristic) for  the  process  at  different  temperatures  and  plotting  the 
above  curve.  If  this  is  rectilinear,  the  value  of  /x  is  constant,  and 
a  temperature  constant  for  the  process  is  obtained. 


BIOLOGY    OF    BIRCH    LEAF    SKELETONIZER 


469 


Fig.   37.     Effect   of   temperature  on   the   fourth    instar.     A   is   the   time- 
temperature  curve,  and  B  is  the  rate  of  development  curve. 


In  calculating  the  value  of  /x  for  the  development  of  Bucculatrix 
the  period  in  the  second  molting  web  and  that  of  the  fifth  instar 
will  be  considered,  as  they  give  two  distinct  phases  in  the  larval 
life,  and  in  this  particular  case  contain  the  data  for  the  most  larvae. 
The  values  are  as  follows : 


Second  Molting 

Web 

Fifth  Instar 

Temperature 

M 

Temperature 

M 

I2°-I4° 

24468 

I2°-I5° 

20644 

14   -21 

23003 

15   -21 

16637 

21    -25 

16396 

21    -25 

14709 

25   -29 

3431 

25   -29 
29  -34 

9694 
—7793 

47°  CONNECTICUT    EXPERIMENT    STATION  BULLETIN    288 

The  values  for  temperatures  above  290  are  included  simply  to 
indicate  the  trend  of  the  curve.  There  is  a  non-survival  of  larvae 
at  these  high  temperatures  and  the  values  are  of  no  significance 
here.  A  steadily  declining  value  of  /x  is  shown,  and  if  a  curve 
of  log  rate  against  reciprocal  of  temperature  were  drawn,  it  would 
be  curvilinear  and  not  straight.  The  data  of  Krogh  referred  to 
above  give  the  same  type  of  curve,  as  the  following  figures,  calcu- 
lated from  them,  show : 


Temperature 

M 

i3.45°-i5-55° 

32989 

15.55  -17.00 
17.00  -18.80 
18.80  -20.90 

25040 
28400 
26673 

20.90  -23.65 
23.65  -27.25 
27.25  -32.70 
32.70  -32.95 

19475 

15362 

7589 

18203 

In  the  case  of  the  last  figure  in  the  column  the  temperature  is 
too  close  to  the  preceding  to  permit  any  significance  to  be  attached 
to  the  value  attained.  The  temperature  characteristics,  although 
showing  the  same  tendency  of  variation,  differ  in  their  absolute 
values  from  those  of  Bucculatrix,  as  would  be  expected.  In  both 
cases  an  increasing  temperature  gives  a  decreasing  value  of  jx. 
The  work  of  Brown  (1926)  on  the  development  of  an  instar  of 
various  Cladocerans,  and  that  of  Bliss  (1926)  on  the  prepupal 
period  of  Drosophila,  however,  show  a  constant  value  of  f*.  over 
considerable  temperature  ranges.  Thus  Brown  finds  the  follow- 
ing characteristics  for  Cladocerans : 


Temperature 

M 

Simocephalus  serrulatus 

150  -24.60 

C. 

16950 

24     -32 

4780 

Monia  macrocarpa 

II     -20 

28500 

20    -27.6 

17210 

27-6-33 

74io 

Pseudosida   bidentata 

14   -27.5 

19800 

Bliss  derives  the  following  values  for  Drosophila : 


Temperature 

M 

12° -16° 
l6    -25 
25     -30 

33210 

16850 

7100 

Crozier  (1926)  has  stated  that  "there  is  as  yet  scarcely  suffi- 
cient evidence  to  verify  the  prediction  that  the  curve  relating  log 
velocity  of  growth  to   i/T,   when  velocity  equals   reciprocal   of 


BIOLOGY    OF    BIRCH    LEAF    SKELETONIZER 


471 


Fig.  38.     Effect  of  temperature  on  the  fifth  instar.     A  is  the  time-tempera- 
ture curve,  and  B  is  the  rate  of  development  curve. 


time  required  to  reach  a  defined  stage,  should  be  slightly  curvi- 
linear. But  there  is  an  indication  that  growth  velocities,  where 
evidenced  as  constant  rates  of  increase,  adhere  satisfactorily  to 
the  Arrhenius  formula ;  and  even  when  we  may  quite  reasonably 
expect  that  an  autocatalytic  system  is  involved,  the  agreement  is 
quite  as  good  as  might  be  desired.  The  values  of  the  temperature 
characteristics  of  growth  phenomena  are  quite  varied,  yet  they 
cluster  quite  definitely  about  the  following  magnitudes  :  7-8,000 ; 
11-12,000;  16-17,000;  20,000;  24,000;  27,000."  He  brings  a 
considerable  mass  of  evidence  to  support  this  view,  and  for  a 
detailed  discussion  his  works  may  be  consulted.  It  is  simply 
desired  here  to  compare  the  values  for  Bucculatrix  with  those  for 
other  animals  and  to  call  attention  to  the  fact  that  different  spe- 
cies vary.     Heilbrunn   (1925)  has  offered  some  criticism  of  the 


472 


CONNECTICUT    EXPERIMENT    STATION  BULLETIN    21 


temperature 

Fig.  39.     Effect  of  temperature  on  the  total  external  feeding  period.     A  is 
the  time-temperature  curve,  and  B  is  the  rate  of  development  curve. 


use  of  the  critical  thermal  increment  of  Arrhenius  in  relation  to 
biological  processes  of  a  complex  nature  and  involving  several 
physical  as  well  as  chemical  reactions,  and  the  identification  of 
basic  biological  processes  by  comparing  temperature  character- 
istics is  open  to  question. 

The  effect  of  temperature  on  the  development  of  the  larvae  of 
Bucculatrix  canadensis ella  may  be  summarized  as  follows : 

The  lowest  constant  temperature  at  which  larvae  will  complete 
development  is  between  io°  and  I2°C,  and  the  highest  tempera- 
ture is  slightly  under  34°C. 

The  curve  of  the  rate  of  development  is  sigmoid,  and  above 
30°-3i°  the  temperature  has  a  depressing  effect. 

In  view  of  the  fact  that  larvae  will  live  for  a  considerable  period 
of  time  at  io°  and  even  develop  somewhat,  although  this  tempera- 


BIOLOGY    OF    BIRCH    LEAF    SKELETONIZER 


473 


""""""""H lH"l[llllllllll 


Fig.  40.  Effect  of  temperature  on  rate  of  development.  W2  represents 
the  period  in  the  second  molting  web ;  4th,  the  fourth  instar ;  5th,  the  fifth 
instar;  t,  the  total  feeding  period.  The  data  are  the  same  as  those  used  in 
making  the  B  curves  in  figures  36  to  39  but  the  plotting  paper  is  arith- 
logarithmic,  and  no  attempt  has  been  made  to  smooth  the  curves.  This 
figure  shows  the  comparative  effect  of  temperature  for  the  different  stages. 


ture  is  lethal  if  continuous,  this  cannot  be  considered  a  physio- 
logical zero. 

Fluctuations  in  temperature  have  their  greatest  effect  on  the 
growth  of  the  larvae  when  they  occur  around  21  °  to  25°. 

The  temperature  characteristic  (critical  thermal  increment)  cal- 
culated according  to  the  Arrhenius  formula  is  not  constant  but 


47  4  CONNECTICUT    EXPERIMENT    STATION  BULLETIN    288 

steadily  declines  in  value  as  the  temperature  increases.  This 
temperature  characteristic  is  not  the  same  for. the  quiescent  pre- 
molting  period  as  for  the  feeding-  period,  and  the  rate  of  its  change 
with  changing  temperature  is  also  different. 


Table  4.     Effect  of  Temperature  on  Development,  1926 
Time  in  Second  Molting  Web 


Larva  No.  Temp.  (C) 

164    12°  (c) 

165    

167    

169    

170    * 

173    

Mean 

251   14°  (d) 

254  

255;   

256  

257  

258  

259  

260  

Mean ■ 

122  200  (e) 

125   • 

127  

128  

226  

235  

131   

132  

133   

134 

Mean 

123   2i"(e) 

129  

135  

136  

137 

228  

220  

230  

231  

232  

233  

234  

281  

28? : . 

283 

285    

286 


Entered  web 

Hours 

(date) 

in  web 

9-21-26 

178 

9-18-26 

130 

9-17-26 

136 

9-24-26 

128 

9-21-26 

III 

9-19-26 

138 

137  (20) 

9-23-26 

96 

9-22-26 

112 

9-23-26 

IOO 

9-22-26 

101 

9-24-26 

95 

9-22-26 

103 

9-22-26 

95 

9-25-26 

104 

101  (5) 

9-12-26 

39 

9-13-26 

54 

9-12-26 

39 

9-13-26 

47 

9-14-26 

41 

9-12-26 

39 

9-  7-26 

40 

9-  7-26 

42 

9-  6-26 

.32 

9-  8-26 

39 

41(5) 

Q- I 2-26 

47 

9-12-26 

33 

9-  8-26 

48 

9-  7-26 

40 

9-  8-26 

48 

9-1 1-26 

23 

9-10-26 

40 

9-II-26 

23 

9- 1 1 -26 

39 

9- 1 I -26 

30 

Q- 1 1-26 

30 

9-13-26 

38 

9-2O-26 

42 

9-21-26 

40 

9-21-26 

40 

9-22-26 

32 

9-2I-26 

40 

BIOLOGY    OF    BIRCH    LEAF    SKELETONIZER 


475 


Table  4.     Effect  of  Temperature  on  Development,  1926 — Concluded 


Time  in  Second  Molting  Web — Concluded 

Entered  web 
Larva  No.  Temp.  (C.)  (date) 

287    9-20-26 

288    9-21-26 

289    9-20-26 

Mean 

237  220  (e)  9-23-26 

238  9-23-26 

239  9-24-26 

240 9-24-26 

Mean 

no 25°(f) 

in  

112  

113  

114  

115  

116  

118 

119  

120  

121   9-  5-26 

Mean 

155   290  (g)  9-  9-26 

156   9  -9-26 

158   9-  9-26 

159   9-10-26 

160  9-  9-26 

161    9-  9-26 

162  9-10-26 

163   9-  9-26 

Mean 

261   ■. 340  (h)  9-17-26 

262  9-17-26 

265   9-17-26 

270  9-17-26 

Mean 

263   350  (h)  9-16-26 

264  9- 17-26 

266  9- 17-26 

268   9-16-26 

Mean 


9-  5-26 
9-  4-26 
9-  5 -26 
9-  5-26 
9-  5-26 
9-  6-26 
9-  5-26 
5-26 
4-26 
4-26 


9- 
9- 
9- 


Hours 
in  web 

49 
40 

49 
39  (7) 

34 

42 

39 
40 
39  (3) 

3i 
32 
15 
28 
28 
33 
30 
2.2 

25 
25 
28 

27  (5) 

23 

25 
25 
25 
23 
3i 
24 
25 
25  (2) 

23 
23 
33 
23 
26(4) 

24 
24 

24 
24 
24  (o) 


Table  5.     Effect  of  Temperature  on  Development,  1926 
Duration  of  Fourth  Instar 


Larva  No.  Temp.  (C.) 

164    n°(c) 

165    

166    

167    


First  molt 

Duration 

veb  vacated 

of  instar 

(date) 

(hours) 

9-  9-26 

465 

9-  7-26 

38l 

9-  9-26 

343 

9-  9-26 

359 

476 


CONNECTICUT    EXPERIMENT    STATION  BULLETIN    288 


Table  5.     Effect  of  Temperature  on  Development,  1926 — Continued 


Duration  of  Fourth  Instar — Continued 


Larva  No.  Temp.  (C.) 

168   

173    

Mean 

251    H°  (d) 

252  

253  

254  

255   

256  

257  

258  

259  

260  

Mean 

128  200  (e) 

131   

132 

133  

136 

281   

Mean 

122  210  (e) 

123  

124  

125  

126  

127 

129  

134  '■ 

135  

137  

226  

228  

229  

230  

231  

232  

233  

234  

235  

237  

238  

239  

240  

282 

283  

285 

286  

287  

288 

289  

Mean , 


First  molt 

web  vacated 

(date) 

9-  8-26 
9-1 1-26 

9-18-26 
9-15-26 
9-15-26 
9-15-26 
9-17-26 
9-17-26 
9-18-26 
9-15-26 
9-16-26 
9-17-26 

9-  9-26 
9-  4-26 
9-  3-26 
9-  3-26 
9-  4-26 
9-18-26 


9-  9-26 
9-  9-26 
9-  9-26 
9-  9-26 
9-  9-26 
9-  9-26 
9-  9-26 
9-  4-26 
9-  4-26 
9-  4-26 
9-  9-26 
9-  8-26 
9-  8-26 
9-  8-26 
9-  8-26 
9-  8-26 
9-  8-26 
9-  8-26 
9-  9-26 
9-20-26 
9-21-26 
9-21-26 
9-20-26 
9-18-26 
9-18-26 
9-18-26 
9-18-26 
9-18-26 
9-18-26 
9-17-26 


Duration 

of  instar 

(hours) 

319 

345 

369  (47) 
231 

239 
217 
281 
263 
225 
240 
264 

255 
281 

250  (21 
144 
105 
143 
in 
120 
105 
121  (16) 

112 

134 
119 
158 
119 
112 
90 

135 
144 
144 
161 

81 

81 

81 

97 

97 

97 
144 
105 
106 
105 
in 
145 
113 
113 
130 
120 
118 
113 
118 
117  (21) 


BIOLOGY    OF    BIRCH    LEAF    SKELETONIZER 


477 


Table  5.     Effect  of  Temperature  on  Development,  1926 — Concluded 


Duration  of  Fourth  Instar — Concluded 


Larva  No.  Temp.  (C.) 

no  25°  (f) 

in 

112  

113 

114  

us  

116 

117 

118 

119  

120    

121     

Mean 

154  • 290  (g) 

155   

157   

158  

159  

160  

161   

162  

163   

Mean 

261   35°  (h) 

262 

263  

264  

265   

266  

268  

270 

Mean 


First  molt 

web  vacated 

(date) 

9-  3-26 
9-  2-26 
9-  2  26 
9"  3-26 
9-  3-26 
9-  3-26 

3-26 

3-26 

3- 

3- 

3- 

3- 


9- 
9- 

9- 
9- 
9- 
9- 


-26 
-26 
-26 


9-  7-26 
9-  8-26 
9-  8-26 
9-  8-26 
9-  8-26 
9-  8-26 
9-  7-26 
9-  7-26 
9-  8-26 


9-15-26 
9-15-26 
9-15-26 
9-15-26 
9-15-26 
9-15-26 
9-15-26 
9-15-26 


Duration 

of  instar 

(hours) 

85 
76 
69 

77 
77 
94 
79 
79 
7i 
64 
64 
75 
76(8) 

7i 
55 
48 
55 
80 

55 
7i 
80 
56 
63(11) 

77 
77 
62 

63 
95 
79 
62 

77 

74  (11) 


Table  6.    Effect  of  Temperature  on  Development,  1926 


Duration  of  Fifth  Instar 


Larva  No.  Temp.  (C.) 

531    10°  (b) 

533  

534  

536  

537  

Mean 

164  120  (c) 

165  

167  

168  

160.  


Second  molt 

web  vacated 

(date) 

9-23-26 
9-24-26 
9-28-26 
9-25-26 
9-24-26 


9-28-26 
9-22-26 
9-24-26 
9-21-26 
9-30-26 


Duration 

of  instar 

(hours) 

567 
452 
738 
587 
604 
590  (9l) 

249 
239 
285 


47* 


CONNECTICUT   EXPERIMENT    STATION  BULLETIN    288 


Table  6.     Effect  of  Temperature  on  Development,  1926 — Continued 


Duration  of  Fifth  Instar — Continued 


Larva  No.  Temp.  (C.) 

170    

173    

542    • 

543  

544  

545   

546  

547  

549  

550  

Mean 

251   15°  (d) 

254  

255  

257  

258  

259  

260  

Mean 

123   200  (e) 

124 

126  

127   

129  

230  

235   

Mean 

122  210  (e) 

125  

128  

131  

132  

133  

134  

135  

136  

137  

226  

228  

231  

232  

233  

234  

237  

238  

239  

240  

281  

283  

285   

286  


Second  molt 

web  vacated 

(date) 

9-25-26 
9-25-26 
9-25-26 
9-25-26 
9-24-26 
9-24-26 
9-23-26 
9-26-26 
9-23-26 
9-23-26 


9-27-26 
9-27-26 
9-28-26 
9-28-26 
9-26-26 
9-26-26 
9-29-26 

9-I4-26 
9-14-26 
9-14-26 
9-I4-26 
9-13-26 
9-12-26 
9-I4-26 

9-I4-26 
9-15-26 
9-15-26 
9-  9-26 
9-  9-26 
9-  7-26 
9-  9-26 
9-IO-26 
9-  9-26 
9-IO-26 
9-16-26 
9-12-26 
9-12-26 
9-12-26 
9-12-26 
9-I4-26 
9-25-26 
9-25-26 
9-25-26 
g-26-26 
9-22-26 
9-23-26 
9-24-26 
9-23-26 


Duration 

of  instar 

(hours) 

278 
284 
357 
339 
259 
250 
379 
403 
267 
3ii 
319  (69) 

220 
226 
211 
187 
213 
229 
242 
218  (16) 

120 
135 
135 
152 
150 
104 
107 
129  (18) 

104 
120 
120 

ii5 
126 

153 

93 
159 
135 

92 
123 

135 
123 

99 

99 

148 

115 

I/O 

153 
122 
129 

131 
126 

94 


BIOLOGY    OF    BIRCH    LEAF    SKELETONIZER 


479 


Table  6.     Effect  of  Temperature  on  Development,  1926 — Concluded 


Duration  of  Fifth  Instar — Concluded 


Larva  No.  Temp.  (C.) 

287    

288    

291    

293    

294    

295    

296    

297   

298 

299   

300   

551    

552  

553  

554  

555   

556  

557  

558  

559   

Mean 

no  25°(f) 

in  

US  

116  

117  

118  

119  • 

120  

121   

Mean 

155   290  (g) 

156  

157  

158  

159  

160  

161   

163  

Mean 

526   340  (h) 

527  

528   

529   

530  

Mean 


Second  molt 

web  vacated 

(date) 

9-22-26 
9-23-26 
9-20-26 
9-20-26 
9-20-26 
9-21-26 
9-20-26 
9-20-26 
9-20-26 
9-I9-26 
9-20-26 
9-24-26 
9-23-26 
9-23-26 
9-23-26 
9-24-26 
9-23-26 
9-24-26 
9-24-26 
"9-23-26 

9-  7-26 

9-  6-26 

9-  7-26 

9-  6-26 

9-  6-26 

9-  6-26 

9-  5-26 

9-  5-26 

9-  6-26 


9-10-26 
9-10-26 
9-10-26 
9-10-26 
9-11-26 
9-10-26 
9-10-26 
9-10-26 

9-22-26 
9-23-26 
9-23-26 
g-24-26 
9-23-26 


Duration 

of  instar 

(hours) 

115 
114 

91 
120 

97 
120 

07 

86 
112 
104 
104 
146 

99 
175 
112 
116 
156 
104 
178 

99 
121  (24) 

92 

97 

97   . 
108 

80 

88 
133 

75 

72 

94  (18; 

77 
55 
80 
55 
82 
81 
65 
65 
70(11) 

90 

9i 
96 
66 
86 
86  (10) 


480 


CONNECTICUT    EXPERIMENT    STATION 


BULLETIN    21 


Table  7.     Effect  of  Temperature  on  Development,  1926 


Duration  of  External  Feeding  Period 


Larva  No.  Temp.  (C.) 

164   12°  (c) 

167    

168   

169    

170    

173    

Mean 

251   14°  (d) 

252  

253  

254  

257  

258  

259  

260 

Mean 

230  200  (e) 

127  

128 : 

129 

132 - 

Mean 

.    122  210  (e) 

123  

124 

125   

126  

131   

133  

134  • • • • 

135   

136  

*37  

226  

228  

231  

232  

233  

234  

235  

237  

238  

239  ■ • • • 

240 

281   

283   

285   

286  

287 

288  

Mean 


First  molt 

web  vacated 

(date) 

9-  9-26 
9-  9-26 
9-  8-26 
9-  9-26 
9- 1 1 -26 
9-II-26 


9-18-26 
9-15-26 
9-15-26 
9-15-26 
9-18-26 
9-15-26 
9-16-26 
9-17-26 


8-26 
9-26 
9-26 
9-26 
3-26 


9-  9-26 
9-  9-26 
9-  9-26 
9-  9-26 
9-26 
4-26 
3-26 
4-26 
4-26 
4-26 
4-26 
9-26 
8-26 
8-26 
9-  8-26 
9-  8-26 
9-  8-26 
9-  9-26 
9-20-26 
9-21-26 
9-21-26 
9-20-26 
9-18-26 
9-18-26 
9-18-26 
9-18-26 
9-17-26 
9-18-26 


Duration 

of  period 

(hours) 

714 
644 
707 
803 
629 
629 

688  (62) 

45i 
476 
456 
507 
427 
477 
484 
'523 
475  (29) 

185 
264 
264 
240 
269 
244  (3i) 

216 

254 
254 
278 
254 
220 
264 
228 
303 
255 
236 
284 
216 
220 
196 
196 
292 
212 
221 

275 
264 
267 
234 
244 
256 
214 

233 

227 
243  C28) 


BIOLOGY    OF    BIRCH    LEAF    SKELETONIZER 


481 


Table   7.    Effect    of    Temperature    on    Development,    1926 — Concluded 


Duration  of  External  Feeding  Period — Concluded 


Larva  No.  Temp.  (C.) 

no  25° (f) 

III  

115  

116 

ii7  • 

118 

119  

120    

121 

Mean 

155   290  (g) 

156  

157  

158  

159   

160  

161   

163  

Mean 


First  molt 

web  vacated 

(date) 

9"  3-26 
9-  2-26 

3" 

3- 

3- 

3- 

3- 

3- 


9- 
9- 
9- 
9- 
9- 


-26 
-26 
-26 
-26 
-26 
-26 


9-  3-26 


9-  8-26 


7-26 
8-26 
8-26 
8-26 
8-26 
7-26 
8-26 


Duration 

of  period 

(hours) 

177 
173 
191 
187 
159 
159 
197 

139 
147 
170  (19) 

132 
Il8 
128 

no 
162 
136 
136 
121 
130  (14) 


Table  8.     Effect  of  Temperature  on  Development,  1926 


Duration 

of  Quiescent  Period  in  Second  Molting 

Web 

Average 

Standard 

Number 

Temperature     Time  in  Web 

Deviation 

Index  of 

Tangent  to 

of  Larvae 

(C.) 

(hours) 

(hours) 

Development 

Curve  B 

6 

12° 

I37±  5-6 

20 

.0072 

8 

14 

I0I±   1.2 

5 

.OO99 

.6770 

10 

20 

4i±  1.1 

5 

.0244 

I.3I00 

20 

21 

39±  1.0 

7 

.0256 

1. 7I0O 

4 

22 

39±   1.0 

3 

.0256 

1.8855 

11 

25 

27±   1.0 

5 

.0370 

I.2800 

8 

29 

25 ±  o.s 

2 

.04OO 

•8350 

4 

34 

26±  1.4 

4 

•0385 

4 

35 

24±    0 

0 

.0417 

.7700 

Duration  of  Fourth  Instar 

Average  Duration 

Standard 

Number 

Temperatu 

re          of  Instar 

Deviation 

Index  of 

Tangent  to 

of  Larvae 

(C.) 

(hours) 

(hours) 

Development 

Curve  B 

6 

11° 

309±I3.2 

47 

.0027 

10 

14 

250±  4.4 

21 

.0040 

•2395 

6 

20 

I2I±    4.5 

16 

.0083 

•4215 

30 

21 

II7±  2.6 

21 

.0085 

.4275 

12 

25 

76±  1.5 

8 

.OI32 

.6265 

9 

29 

63±  2.5 

11 

.0159 

.1240 

8 

35 

74±  2.6 

11 

•0135 

.3860 

482 


CONNECTICUT    EXPERIMENT    STATION  BULLETIN    288 


Duration  of  Fifth  Instar 


Average  Duration 

Standard 

Number 

Temperature 

of  Instar 

Deviation 

Index  of 

Tangent  to 

of  Larvae 

(C.) 

(hours) 

(hours) 

Development 

Curve  B 

5 

10° 

590±274 

91 

.0017 

15 

12 

3i9±i2.o 

69 

.OO31 

.2885 

7 

15 

2i8±  4.1 

16 

.OO46 

.2610 

7 

20 

I29±  4.6 

18 

.0077 

•3S80 

44 

21 

I2I±    2.4 

24 

.0083 

•3725 

9 

25 

94±  4.0 

18 

.OI06 

.4280 

8 

29 

7o±  2.6 

II 

•0143 

•2935 

34 


86±  3-0 


10 


.0116 


•5956 


Duration  of  External  Feeding  Period 


Average  Duration       Standard 

Number 

Temperature 

of  Period 

Deviation 

Index  of 

Tangent  to 

of  Larvae 

(C.) 

(hours) 

(hours) 

Development 

Curve  B 

6 

12° 

688±I7 

62 

.0OI5 

8 

14 

475  ±  6.9 

29 

.0021 

■isrs 

5 

20 

244±  9.3 

31 

.OO4I 

.1760 

28 

21 

243±  3-6 

28 

.0041 

.1895 

9 

25 

I70±  4.3 

19 

.OO59 

•2235 

8 

29 

130C+:  3.3 

XII. 

14 

Control 

.0077 

.1855 

The  control  of  these  larvae  is  a  very  simple  matter  on  orna- 
mental trees.  The  trees  should  be  sprayed  about  the  middle  of 
August  with  lead  arsenate  at  the  rate  of  3  pounds  of  powder  to 
100  gallons  of  water.  Add  one  pound  of  casein-lime  to  aid  in 
spreading  the  poison  and  making  it  adhere  to  the  foliage.  The 
larvae  feed  on  the  lower  side  of  the  leaves,  and  this  side  must  be 
covered  with  the  arsenate.  Experiments  conducted  by  the  writer 
have  shown  that  if  the  trees  are  carefully  sprayed  there  will  be 
practically  no  feeding  by  the  insects. 


XIII.     Summary 

The  history  of  the  genus  Bucculatrix  up  to  the  description  of 
the  species  canadensisella  Chambers  has  been  briefly  reviewed, 
and  an  account  has  been  given  of  the  periodic  abundance  of  this 
species  in  North  America  up  to  the  present.  Systematically  the 
genus  is  usually  placed  in  the  family  Lyonetiidae. 

A  brief  description  is  given  of  the  external  morphology  of ,  the 
different  stages. 

There  is  but  one  generation  a  year  of  B.  canadensisella.  The 
adults  emerge  from  the  cocoons  in  June  and  July  and  oviposit 
on  the  leaves  of  birches.  The  incubation  period  of  the  eggs  aver- 
ages 15  days.  The  larvae  mine  in  the  leaf  during  the  first  three 
instars,  the  mining  period  averaging  between  24  and  31  days. 
The  last  two  instars  feed  externally  on  the  under  side  of  the  leaf, 


BIOLOGY    OF    BIRCH    LEAF    SKELETONIZER  483 

skeletonizing  it,  and  this  feeding  period  averages  from  13  to  15 
days.  The  total  larval  life  averages  from  38  to  46  days.  The 
cocoon  is  spun  on  the  under  side  of  debris  on  the  ground,  and 
hibernation  occurs  in  the  pupal  stage.  The  last  larvae  are  found 
in  the  field  the  latter  part  of  September.  There  are  five  larval 
instars. 

The  number  of  larval  instars  was  determined  by  applying 
Dyar's  hypothesis  to  the  width  of  the  head  capsules. 

The  principal  larval  food  plants  are  the  gray,  paper,  yellow, 
and  European  white  birches.  Some  feeding  on  black  birch  has 
been  observed. 

The  Hymenopterous  parasites,  of  which  ten  species  have  been 
reared,  and  the  ants  and  other  predaceous  foes  are  the  principal 
factors  affecting  the  abundance  of  this  insect. 

The  geographical  range  includes  southern  Canada  and  northern 
United  States,  the  insect  being  particularly  abundant  around  the 
Great  Lakes  and  east  to  the  Atlantic  Ocean. 

Temperature  has  a  marked  influence  on  the  development  of  the 
larvae.  At  io°C  and  lower,  and  at  340 C.  and  higher,  they  cannot 
survive.  The  curve  obtained  by  plotting  rate  of  development 
(reciprocal  of  hours  taken  to  complete  a  given  stage)  against 
temperature  is  sigmoid.  Above  300  temperature  has  a  depressing 
effect.  An  absolute  physiological  zero  was  not  obtained  because 
of  the  ability  of  the  larvae  to  develop  slightly  at  a  low  tempera- 
ture which  was  fatal  if  continued  a  sufficient  length  of  time. 
There  is  no  thermal  constant  for  any  temperature  range  beyond 
the  experimental  error,  and  the  temperature  characteristic  as  com- 
puted by  the  Arrhenius  formula  steadily  decreases  in  magnitude 
as  the  temperature  increases. 

The  use  of  a  lead  arsenate  spray  about  the  middle  of  August 
will  protect  the  trees  against  injury  by  the  larvae. 

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PLATE  XVII. 


a.    Adult   (left)  and  eggs  (right)  of  Bucculatrlv  canadensisclla  on  birch 
leaves.     Adult  enlarged  eight  times,  eggs  enlarged  ten  times. 


b.  Vacant  mine  of  larva  in  birch  leaf  (left)  and  fully  grown  larva 
(right)  with  first  (lower)  and  second  (upper)  vacant  molting  webs. 
Enlarged  four  times. 


BIRCH   LEAF   SKELETONIZER 


PLATE  XVIII. 


a.     Larva  of  Bucculatrix  canadensisclla  skeletonizing  leaf  of  gray  birch. 
Slightly  enlarged. 


b.     Cocoons  of   Bucculatrix  canadensisclla  on   dead  leaf.     Enlarged   four 
times. 

BIRCH    LEAF  SKELETONIZER 


University  of 
Connecticut 

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